Being allowed to return to live with parents after uni is a privilege, not a right. So I would say that you do all you can now to support him, try arrange support, and when he's at uni if he wants to live with you again you make it clear that would be dependent on him seeking treatment for his mental health, and it improving, or you will support him to find his own place.
People will say that's harsh because nobody can afford to live alone these days but I don't know many people ever who've moved from parents to uni to their own place, people tend to get a house share which is far more affordable.
Mental health problems can explain behaviours. It can't justify them, unless we're talking somebody disconnecting from reality in a psychotic episode for example. He's nearing adulthood and needs guidance to take on responsibility for his own health and wellbeing. You can guide and support, but you don't have to tolerate poor treatment.
For disclosure, I never returned 'home' after uni, always stood on my own feet, and learned a lot from it. I'm glad, compared to seeing the journeys of friends who went from uni to their parents and then often stagnated for years. I think a lot of parents find it hard to accept their kids are adults, when they are adults, and still feel that they are children that need to be protected and caught when they fall. You can do that within reason but you don't have to sacrifice the sanctity and stability of your home. You matter too, you know?
I've also experienced crushing depression, three episodes now for a year or two apiece, so I know what it's like to be in that place where your mind is telling you 24/7 you'd be better off not here, nobody loves you, they might pretend they want you to stick around but it's just cos they don't know any better and actually when you're gone they'll find out it's much better. That you should hurt yourself, because it's what you deserve, never finding joy or peace in anything, time moving like a snail because you just feel so utterly wretched every second, mind and body, it's like the universe is punishing you by making you experience every split second ten times over. I get it. I wouldn't wish it on my worst enemy.