He originally complained of tiredness and lack of appetite, and his father, Craig Milne, says doctors still don't know what has damaged his son's heart - but only know it is too weak to function.
His family, from Aston in Sheffield, are now waiting for the phonecall to say a donor has been found to save his life.
"He’s such a good lad,” Craig told our sister paper The Star. “He’s very popular. He’s got lovely manners – before he went into his coma, the nurses and staff said ‘you should be proud of him, he’s so polite’
"He loves his football and supports Liverpool. We go out walking with the dogs often and go camping or caravanning in Derbyshire. We just have to leave it in the doctors’ hands. He’s in the best place he could be.
"It’s frustrating as a parent, not being able to do anything, but that’s the way it is.”
Craig said he took Kallum to the doctors two weeks ago when his lad was suffering from fatigue and went off his food.
The GP initially prescribed iron, but days later Craig noticed the 11-year-old’s feet and lower back had swollen.
Craig said: “At first I didn’t think too much of it and ‘low iron’ sounded right. But when the swelling started it got more serious.
Things moved rapidly from there. A trip to a walk-in centre became an immediate referral for a two-day stay in Rotherham Hospital.
When specialists realised how poorly Kallum was, he was stabilised and transferred to Leeds Infirmary, only for doctors to order him to Great Osmond Street Hospital in London for critical care.
The 11-year-old was placed in a medically induced coma on April 27 and is now being treated with a ECMO mechanical heart while waiting for an emergency transplant.
Craig said: “They put him in a coma to give his heart a rest, but when did more tests they realised it had more or less stopped it was that weak. And they just can’t find what caused it.
"He’s that poorly he’s on top of the waiting list for a transplant.”
Meanwhile, Craig, a self-employed groundsworker, has been travelling between Aston, Leeds and London to be near his son.
Family friend Jason Sharpe has since set up a GoFundMe page to pay Craig’s expenses, which has raised over £2,600 in two days.
Jason said: “Obviously with the situation that it is, it’s not going to be a quick process. It’s going to be quite long and I didn’t want him to be thinking about money.
"Kallum’s a brilliant lad. He’s the type of boy who always asks how you are, and that’s not common for an 11-year-old.
"He’s a good friend to me son. He’s a good, cracking lad and it’s really distressing what’s happened.”