The things I learned when facing a possible mental health crisis with H in my sitch: all you can do is let people know you're concerned, and then step back.
Right now your H is agitated with you for whatever reason. Don't give him additional ammunition. Simply say, "I'm unable to mail you the prescription." And leave it at that.
If you've notified his parents or friends and expressed your concern for his well being, the only other options you would have is to research and see if there's some sort of mental health mobile crisis team that could evaluate him.
You can't fix it. And you can't help him if he can't help himself. Set up/enforce boundaries, and take care of yourself at this point in time.
M:32,H 32 T:10, M5 BD/H Move Out: 9/2014 - extreme anger H Mental Illness Diagnosis: 4/15 Served D Papers: 10/15 Divorced: 11/15