Sleeping all day sounds like it could possibly have something to do with a physical cause. Has your son ever had his thyroid function thoroughly evaluated? How are his eating habits? Are his vitamin D levels good?
Or does he sleep all day because he stays up all night, either working or socializing or . . . watching movies? Those last two would be a bit irresponsible, in my view. If that's not the case, though, maybe there is a doctor who would be interested in finding out why he can't stay awake?

I wouldn't worry that your 'enabling' is handicapping him. There are a lot of people with problems like his who are simply on the streets because no one close to them can or will take care of them. It is especially helpful that you are giving your grandson some stability that he might not have otherwise.
Oh, I just saw in another thread that he is vegetarian. Has he had his iron and serum ferritin checked? Men can get low on iron, too, not just women, and low iron can sometimes be associated with fatigue, even if he doesn't actually have anemia.
So can sub-optimal levels of zinc, selenium, iodine, and especially the B vitamins.
I also saw in the other thread that your daughter-in-law is also living there? Would you feel comfortable approaching her with your concern about your son sleeping a lot? If she is also concerned, maybe both of you together could convince him to talk to a doctor about it?
I imagine they might both be slightly offended by any suggestion that their vegetarian diet could be contributing to his daytime sleepiness. What do you think?