Covid can pretty much cause anything. I have a friend who was training for an ultra marathon pre covid with no health issues. She got covid, all she had was fatigue at the time (the April of first lockdown).
She is now in a wheelchair, had her bladder removed and has pretty much been through hell for 2 years, in an out of hospital. There is progress but it's slow.
So it's plausible that his body has reacted badly to covid and it has caused further issues and damage but I doubt we will ever know, and most have just moved on from covid so probably won't even be considered and he will just be called a sicknote.