The mother of an autistic man who allegedly wanted to shoot Donald Trump at a campaign rally before the US election has appealed to Barack Obama to allow her son to serve his sentence in Britain.

Michael Sandford, 20, will be sentenced later this month after pleading guilty to lesser charges of being an illegal alien in possession of a firearm and disrupting government business.

The Briton attempted to grab a pistol from a police officer at a Trump campaign event in Las Vegas four months before the election, and said he had wanted to kill the then presidential candidate.

Sandford had hoped to receive the minimum sentence of 18-24 months in prison, because he has no previous convictions, and to be allowed to serve it in England. But his mother has said he is terrified that pressure from the president-elect may result in a more severe sentence that has to be served in the US.

Lynne Sandford said: “When he first heard that Mr Trump had won, he told me: ‘I’m so glad I didn’t do what I wanted to do that day, and I do accept and respect the decision of the American people.’ But within 24 hours, it hit him like a sledgehammer.

“He was in a terrible state, saying: ‘What if the judge decides to increase my sentence to get onside with Trump? What if Trump wants to have me bumped off in prison, what if he wants to make example of me and not let me out of prison?’”

Lynne Sandford has seen her son once since he was taken into custody, but will fly to the US for his sentencing on 13 December. She said Michael had recently written her a letter saying he was struggling to get by.

“He said: ‘I can’t describe how I’m feeling. Tears are streaming down my face as I write this. I miss you all and love you all more than words can explain. I need to be back home with you all. I’m so alone, cold and scared here. You all mean everything to me and I’m so sorry for how I have treated you all. I love you, I love you, I love you,’” she said.

Her son has a range of acute and complex medical issues. He has had severe obsessive compulsive disorder, was sectioned aged 14, previously tried to kill himself, and has a heart condition. He was diagnosed as having been under the influence of a delusional, psychotic episode at the time of the incident.

He was initially told he faced a maximum sentence of 30 years in prison after being charged in June with disrupting an official function and two firearms offences. He is being held in custody at a US government-run detention centre in Nevada.

“The US and UK have to agree to him serving his sentence in England. I don’t think that’s a problem for the English side, but once Mr Trump is president, he might not agree to it,” Lynne Sandford said. “That’s what we’re worried about.

“I have written to President Obama and Theresa May to see if they can intervene. I’m hoping that President Obama will show compassion and realise that this poor sap could be on the receiving end of Trump’s anger, and I am urging him to act in any way he can while he is still in power.”

She said she had received a standard reply from Downing Street and was yet to hear from the White House. Her son has been on suicide watch for much of his time in detention.

“When he’s on suicide watch, he’s in isolation his cell 24 hours a day. He is not allowed anything in with him; not even toilet paper, in case he rams it down his throat,” she said.

She has been told her son is being given 15 tablets a day, but not what for or what dosage. “All I know is he is being given them erratically, sometimes in one go. He’s had more seizures and has been hallucinating. I would imagine this is the result of the erratic medication,” she said.

Her son had lost a great deal of weight and was less than eight stone, she said. Michael, who is 5ft 10, has had anorexia nervosa in the past. “He is eating porridge in the morning and a bread roll in the evening, and that’s it. He is unable to sleep at night because the lights in his cell are kept on all night. He is severely depressed,” the mother said.

She said she wanted to thank Guardian readers who had donated more than £10,000 to a Crowdjustice page set up to raise money for his legal fees after the Guardian’s Weekend magazine wrote about his case.

“It was an amazing response,” she said. “We were so touched and appreciative, and Michael passes on his sincerest thanks to everybody trying to help him. I’m glad readers realise that despite what he attempted to do, he’s not a bad person at heart, and that he needs help.” She has started a new Crowdjustice page to raise funds to try to bring him back to the UK to serve his sentence close to his family.