I think if it's your kids lives here, you need to just say that there is severe suspicion of drug use and that you want him tested, then volunteer for a test yourself. The courts should do it for children. My friend just went through a custody battle and the courts drug tested both without either of them having history of drug use- it was just protocol.
I think if your gut tells you he was and is using then I don't think it matters what he is using. It sounds more like It could be just pills, but if he has a lot in his system that he's not prescribed then it should be a problem for the court. I would try to pull all the stops out if your kids are in danger to be alone with him.