The real question is whether he’s got the psychology of a hoarder or if he’s just apathetic due to depression and this is just a side effect.
You can clean up a hoarder’s lair, and you might even get them to agree it’s necessary, but without external help and counseling it’s just going to go back to the way it was, and much faster than you’d think possible.
If you honestly think he doesn’t have that hoarder mentality and that having a respectable place would pull him out of his slump then by all means do that. The advice above for getting a junk hauler to come help toss stuff is the way to go. He’ll probably need someone to prod him to decide what’s actually worth keeping (otherwise 90% of the crap will be “worth keeping” and all that will get tossed is a token gesture).