Instinctively, I want to say: stop paying his rent. But, I also understand how frustrating it must be to live several hours away from him and have him end up being homeless. Obviously moving back home is not a great plan for anyone at this point. He would be even more frustrated with himself I imagine.
Can you arrange some sort of contact with his doctor or therapist and ask them how you can best help him?
My mom always felt bad for oldest nephew (his mom kicked him out at age 16) and intermittently paid his bills, lent him money (never paid back), and let him live with her. He is 25 now, and has off-and-on taken his medication, and has settled down with a gf and a little boy.
My younger nephew moved out around age 19, was medicated for the majority of his childhood and has figured out this adulting thing pretty quickly. His parents did not bail him out. He has held down the same job for over 6 months now, and is doing pretty good overall. (He is now 22)
(I tell you this to give you hope that he will get through this transition to adulthood! Both my nephews have ADHD and are doing quite well now!)
Anyway, if you can consult with your son’s support team where he lives, they might now best how to deal with him. I feel like continuing to pay his rent is allowing him to not have to fully face the reality of his life choices. Can you attach some strings to it? Like, if he isn’t taking his meds everyday, he doesn’t get any financial assistance?