The wife of the UK's longest known coronavirus patient says she fears he has "given up" after his condition worsened and he started suffering "fainting attacks".
After making "huge strides" in his recovery in recent months, Mr Kelk's wife Sue had started making plans for his return home by launching a crowd-funding appeal to help convert their property.
But Mrs Kelk told Sky News that in the past week, her 49-year-old husband has deteriorated and has suffered "fainting attacks" - which he has never experienced previously.
She is now worried her husband no longer "believes in himself" in his fight for recovery.
"He's having quite a few problems," Mrs Kelk said.
"A couple of times he's had like a faint and lost consciousness but they don't know why.
"He's a bit more confused, he's not sleeping properly.
"He's not interacting with people on the internet like he was. It's really confusing."
Mrs Kelk added: "I suppose in the bigger scheme of things it is a little blip. But the impact it's had on Jason is absolutely massive.
"I don't think he believes in himself anymore. He's never, ever lost belief in himself. He's always thought positively.
"It's not like that now. He's frustrated.
"I think he knows what's going on but it's almost like he's given up... it's just really hard."
Mr Kelk had spent several weeks off a ventilator in recent months, but he has needed to use one again after his condition worsened and he still requires kidney dialysis, his wife said.
Doctors now believe the primary school IT worker may always need a tracheostomy tube to remove fluid that builds up in his throat and windpipe, Mrs Kelk added.
"He's really frustrated because he's had a look at how he can be but he's obviously stepped back," she said.
"I've been told he'll probably never be able to have his tracheostomy tube out.
"He's literally back to where he was at the end of March and beginning of April."
Before his condition worsened, Mrs Kelk said her husband had started drinking cups of tea and eating soup and was using Facebook Messenger "virtually every single day".
But she said when she last spoke to her husband he was "talking absolute gobbledygook".
"When he actually got out what he was trying to tell me, he was on about a Greggs breakfast - completely random, completely out of context," Mrs Kelk said
She added that her husband - who has lost six stone while in hospital - had also been waking up "confused and thrashing around".
"In a week everything's gone back steps," Mrs Kelk said.
Mr Kelk was admitted to hospital on 31 March last year, around the same time as Derek Draper, the husband of TV presenter Kate Garraway.
Mr Draper was also left seriously ill after contracting COVID but has since returned home after a year in hospital.
Mrs Kelk has launched a GoFundMe appeal to raise money for an extension to her home so she can accommodate her husband's care when he leaves hospital.
The crowd-funding page can be found here.