there is a difference between the doctor saying you have cancer, and him saying: you are losing blood but we don't know where it is going.
there is no bleeding and it isn't leukemia (or other blood cancer), the doctor says: 'your blood is literally disappearing, like poof and its gone'.
so what would one do in such situation? if you have money or skills or connections, the answer is everything available. from trial medicine, to natural medicine, to supernatural rituals, whatever they can think of. people do everything to live if they can.
now in his situation, fantasy, what would one do? try every single available magical healing and similar spells on himself and then any medicine/pill/herb/whatever, trying to see what do good and what do harm/nothing, to the point he would be sick of it and as broken as one can and maybe even die trying if he really wants to be cured.
dispel would not be the first thing on the list, sure, but it wouldn't be that far down on it. he should have the means, he doing it or knowing someone to cast it or where to find it, and he would do it soon while he has still power (like when he confirms he is really losing HP) and not waste it all the way to almost 'noob level' on some random quests. one for sure wouldn't wait without doing nothing and only sit around, right?
but this is whatever, the author can do anything they want and that is that, but it still will feel off for quite a few of us since there is some questionable choices there and there was simple not enough explaining.for me particularly i took it will a bit of salt, sure doesn't make sense, but it is there basically to be the starting point,. without it he would still be the 'hero employee', hopefully i don't find much more questionable choices later on, but if it keeps appearing i will simple move on.