Please don't rely on his "potential" - he has told you what he is going to do, accept his word on that. He is going to drink and enjoy his huge collection of porn. If by some magic he changes, well you can perhaps revisit that at the time.
Addicts choose their addiction all the time. They love their addictions more than anything, it is the focus of their lives. Until such time as he chooses to stop his addictions, for himself and makes that decision for himself, there is not much you can do. You didn't Cause it, can't Control it and can't Cure it (the 3 c's).
With him being so hostile and mean to you, do you think it's a good idea to keep in contact with him at all? Maybe it's better for everyone involved to create more distance?
I would contact a different attorney, once that specifically deals with international law or a large firm that deals with divorce. Or, if that's not an option, perhaps hire a paralegal to do the paperwork. I doubt him being in Japan has anything to do with it. You just file as you normally would.
I'm pretty sure you can file for divorce in the U.S., it may not be recognized in Japan (but I can't see why not), but you don't even need to divorce where you got married. I have been married in one country and divorced in another.