Are you sure the boy (and his family) have done 100% full disclosure of their past? And just how big a “mental health issue” are we talking? Was it medication to calm your nerves during a challenging stage in life? Or is it medication that you need to be on daily?
Sad to say, the world we live in is a pretty small one, so if its something that was really over the top, chances are the shadchan, his parents, or he himself already know, so don’t let it eat you up.
That said, by the time you see “the question” on the horizion, be sure you tell him. Nothing sinks a ship faster than withholding info from your spouse (to be)