In terms of how he is when he is not on the medication, is there a way to find out? Does he have a psychiatrist or psychologist that he would give you permission to speak to? It might be kidai to find out. I have gotten permission to speak to potential shidduchim’s therapists.
Also, try to get a sense for how responsible he is for taking his medication. Also, you should try to find out what he is like while taking the medication. The medication might not solve all his problems, so that is something you would want to find out.