Douglas Gresham remembers well the day that JFK was shot. The news from Dallas had stunned pupils during dinner at his boarding school in Surrey. "It was a dark and drizzly afternoon when someone burst into the dining room and said, 'They shot Kennedy'," he recalls. "We didn't believe it."

Gresham, who had turned 18 that month, joined a growing huddle around the school's television. Soon the reports of JFK's assassination – at 6.30pm London time – were confirmed. "It was a cruel blow," he says. "Most of us regarded him as a great hope for Western civilisation. But it was only the first blow of the day. Soon any thoughts about Kennedy would go completely out of my mind."

A little over an hour later, Gresham was doing homework inside a classroom away from the main school building when an unusual sound alarmed him – the urgent steps of high heels coming down the path outside. "I have no idea why, but I knew immediately it was for me," he says. "Pam, the headmaster's daughter, had come to fetch me. My stepfather had died."

Gresham, who is now 68, knew the man who was his only parent at the time as Jack. The rest of the world knew him as CS Lewis. He died, aged 64, less than an hour before JFK was shot, and 12 minutes after Aldous Huxley succumbed to cancer. News of the writers' coincidental demise would be swallowed by the outpouring of shock and grief springing from Texas. Nobody noticed that Lewis was gone, but for Gresham the loss was devastating.

"I didn't think much about the timing," he says. "I didn't intellectualise the whole thing. It was a situation of being numb and carrying on, irrespective of what was going on in the wider world. Because my world had suddenly become very small and I ceased to be interested in anything else. Grief is a selfish thing – it shuts out everything except your own pain."

Gresham now lives in Malta but has travelled to London to attend a service this afternoon at Westminster Abbey. A stone will be unveiled in Lewis's honour on the floor of Poets' Corner. Using the baritone of a film-trailer voice-over artist, Gresham will read a passage from The Last Battle, the final novel in The Chronicles of Narnia, the books for which Lewis is best remembered.

Gresham was born in New York to the American writers William Lindsay Gresham and Joy Davidman. Their marriage became unhappy and in 1950 Davidman began corresponding by letter with CS Lewis, whose work she admired. They met two years later and, in 1953, at the start of a slow-burning relationship that would inspire the film Shadowlands, Davidman moved to England with Douglas to live near Lewis at Oxford.

Gresham had read the Narnia books that had been published by then, never dreaming that he might be adopted by their author. He had been captivated by Lewis's imagined world, which also fuelled a fantasy about the man who would be his father. "I was an eight-year-old American boy steeped in the medieval legends of King Arthur," he recalls. "England to me was a land where I expected everyone to ride chargers and joust whenever they met. So when I was taken to meet the man who was on speaking terms with the great lion Aslan, I subconsciously expected him to be wearing silver armour and carry a sword.

"But he was the antithesis of what I had imagined – a stooped, balding, professorial gentleman with unbelievably shabby clothes and nicotine-stained fingers. It was also clear, however, that he had an enormous personality and sense of fun. This immediately eroded any visual deficiencies. I lost an illusion and gained a great friend and, later, a father."

Clive Staples Lewis – Jack to his friends – was born in Belfast in 1898, schooled in Hertfordshire, and found his home in Oxford, where he began his studies at University College after fighting in the trenches during the First World War. Narnia came much later. He wrote seven books between 1949 and 1954, by which time he had become a poet, theologian and an academic of renown. He had not married when he met Davidman and, by then in his fifties, warmed quickly to family life.

"When I was home from school, we would get together at meal times and go for walks," Gresham says. "He was quite prepared to come romping around in the woods with me and expect fawns to step out from behind trees at any moment. He was full of laughter and jokes and stories. He said himself he wasn't good with children but I've rarely met a man who was better."

In October 1956, Gresham's mother broke her leg in a fall and was diagnosed with bone cancer. After a period of remission, the disease returned and Davidman died in 1960, shortly after a holiday with Lewis to Greece, where she had always wanted to visit. She was 45. Devastated, Lewis wrote A Grief Observed under a pseudonym. The collection of reflections includes the line: "It doesn't really matter whether you grip the arms of the dentist's chair or let your hands lie in your lap. The drill drills on."

Gresham was 14, and would live with Lewis for the last three years of the writer's life. "He was continually suffering from grief for the first six months, as was I," he recalls. "But then it started to fade into a numb sensation and our relationship slowly normalised. He was never quite the same but there was a contentment – an acceptance of what had happened and, later, what was happening to him."

A year after his wife's death, Lewis became ill with blood poisoning but then recovered fully before his condition deteriorated severely in the summer of 1963. Now Gresham's only guardian, he protected the teenager from the full extent of his poor health. "I'd known that my mother was dying," he remembers. "I had watched her with cancer for five years, but nobody told me Jack was dying.

"The closest I came to understanding was when I met JRR Tolkien outside the hospital. I was coming out as he was coming in to visit Jack. They were close friends. He approached me and said: 'Hello Douglas. You probably won't remember me. I'm Professor Tolkien.' He let me know that if anything happened to Jack and I had nowhere to go, I could come and live with him. I was moved but it still didn't click in my mind that this meant Jack was dying."

The news of Lewis's death, delivered in the headmaster's drawing room by his wife, who clutched a large whisky, came as "a crushing blow", Gresham says. "For the second time in my life, everything had been swept away."

Jill, the headmaster's daughter who had fetched him from the classroom, asked the teenager if she could do anything to help. "I knew that when I got back to the dorm the guys would ask awkward questions, and that I would be on the verge of shaming myself by weeping. So I asked her to tell them first. When I got back nobody said anything, but when I picked up my pillow to get my pyjamas, I found a half bottle of brandy. One of the lads, a friend who I fought with all the time, had got out of school and rushed to the pub. There were four of us and we proceeded to get plastered."

Sober the next day, Gresham took on the task of telling a distracted world about the death of CS Lewis. The writer's older brother and closest friend, Warren "Warnie" Lewis, was devastated and, Gresham recalls, had turned to the bottle "for anaesthesia for the pain he was suffering. It was up to me to manage things. I was 18 and I didn't know what I was doing."

He called Walter Hooper, who had worked with Lewis and later became his literary executor, to ask him to issue a press release. A journalist at the Oxford Mail was one of only two who responded. "In a way, it was good because it gave me a lot of peace," Gresham says. "And because Kennedy's death was so suspicious, the distraction went on. People only very slowly became aware of Jack's death. For years afterwards, his estate would forward letters to me that were still addressed to him. Even today, a child will write to him every now and then."

Gresham went to agricultural college after leaving school and met his wife, Merrie, to whom he has now been married for 47 years. In 1967, they moved to Tasmania. Like Lewis, they found great meaning in Christianity, and for a time they helped lead a counselling missionary in Ireland before settling in the sun in Malta. They have five children and 11 grandchildren, all of whom have stepped through the Narnia wardrobe. Gresham, who is a character in Shadowlands, the 1993 film starring Anthony Hopkins as Lewis, was an executive producer on the three recent Narnia films, and made a cameo appearance in each.

Just as news of Lewis's death was overshadowed so, too, to a lesser extent, has coverage of the 50-year anniversary. But Gresham says his stepfather would have approved. "I think he'd enjoy the idea, to be left alone and not bothered," he explains. "He'd rather his spirit were remembered than his body, and anniversaries weren't a big thing for him, or for me. Merrie and I have forgotten almost all of our wedding anniversaries."