Heart failure - he had myocarditis in 2014 which left him with severely reduced heart function. They fitted an implanted defibrillator which let him live an almost normal life for a few years, but in early 2019 he had a massive heart attack and spent about six months in and out of hospital (including at least two nights in ICU when no-one really expected him to come out again). And since then his blood pressure has to be kept incredibly low for his heart to be able to handle it, which leaves him with no energy at all, and the other medications he takes make him tired and affect cognitive function.