Paul Harvey has dementia, and he wasn't sure if he'd remember how to play one of his old compositions. But when the 79-year-old sat down at the piano, the song came back to him. His son, Nick, captured the touching moment on camera.

Nick, who is from East Sussex, U.K., told CBS News his dad wrote the song "Where's The Sunshine?" in 1981. "[It] was written for an original Imberhorne School (East Grinstead) production... when dad was head of music," Nick explained in a tweet.

He later found sheet music of his dad's work, with lyrics written by his colleague Pete Talman, who was the head of drama at the music school. "It was a fantastic show. I remember it as if it were yesterday," Nick wrote.

Dad has dementia. Sometimes he drifts into another world and I feel like I’m losing him. He is never more present, however, than when he plays the piano. He came to mine today and I asked him to play one of his compositions. He thought he wouldn’t be able to remember it. pic.twitter.com/EQGcXBwB3w — Nick Harvey (@mrnickharvey) June 23, 2019

Although Nick found the sheet music, his dad didn't need it — he played the whole song from memory, despite his dementia. After Nick posted video of his dad playing the piano on Twitter, it quickly went viral. "He is incredibly touched by everyone's response and would like me to thank you all on his behalf," Nick said.

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The video also gained widespread media attention, as well as attention from the Alzheimer's Society. When they reached out about the video, Nick came up with an idea and responded to the organization: "How about (dad willing) we get him to record his piece on a nice grand in a nice hall with nice string players and then release it as a charity single for you?"

Nick later tweeted that his dad was willing to record the song for charity — and so was producer Jake Jackson, who Nick described as "one of the U.K.'s top recording engineers/producers has just tweeted this."

Soon, another famous musician joined in. Cellist Peter Gregson asked if he could be a part of the recording.

The plan to record his dad's song for charity got Nick thinking. "I wonder if the string players might also be old ex-professionals with dementia," he tweeted.

This song was so important to Paul, he kept it in his heart after all these years. And now, it could be important to other people with dementia — as a charity song to benefit the Alzheimer's Society. In the meantime, Nick urged people to donate to the Alzheimer's Society. "They do so much to help people living with dementia, and every donation really does make a difference," he wrote.