Would it be possible to put a provision in the divorce decree that if he is arrested at any time for drinking/using when the children are in his custody that the custody arrangement be immediately revisited? If he refuses to do this, it may be interesting to have him explain to a judge why he objects. After all, if he is such a gent and would never do such a thing, then why would he care?
This is an interesting suggestion. I'm a big fan of having everything in the custody agreement that you can think of, because once it's over you don't want to have to go back if it can be avoided. If for some reason you get to mediation instead of a custody hearing, I would definitely throw something like this in the agreement. See what your lawyer thinks.
Good luck. You're doing really, really well. This is hard, and it sucks, but it will end.
xo