Since no one has yet chimed in, I just thought I'd say congrats on keeping them so long. If your profile pick is from the batch of F1 wild greens, it looks like they've been really nice fish. I'm a fan of wilds and wild-types.
Personally, I don't think it is cruel to keep him alone if his partner passes. You could consider QTing some slow-moving dithers to share space with him if you think some extra stimulation would help him, but at this point he knows his (and your) routines and is unlikely to be skittish about anything, it seems to me. I imagine if discus turn out to be like some other pets that will give up the ghost after their owners or companions pass, then he will naturally let himself go without you needing to do anything. And your intuition might be correct, though I suppose he could surprise you if you get to a point where you want to try something like putting him with the young'uns. (Maybe he'd get a second wind.)
At the end of the day, you know him best, though maybe someone else who has kept geriatric discus will share any stories or experiences they have had.