I can see both sides.
However, looking from the outside in (which is all I can do ;o) I think that he was right to go out. If you have this illness which the medical profession can do nothing about, then him taking some 'time out' is good for him in order for him to be able to cope ~ after all, by your admission he is doing a lot for you and looking after you the majority of the time.
I do sympathise with you, but believe that him wanting to go out once a fortnight is no real biggie in the great scheme of things. Yes, cut him some slack. Everyone needs a little TLC sometimes ~ he has done it for you, so let him have some too :o)