Addiction might explain his crazy behaviour - not a particular substance. Or he may just act crazy for no particular reason. No-one can really say. JG is right on the money - you will drive YOURSELF crazy trying to figure out what makes him tick and why he does the things he does. The fact of the matter is that his behaviour is affecting you in a very negative way.
I'd say there's a good chance that he has not been honest with you about his using, but I think deep down you know that anyway. You've gotten some good advice here, PW. I know it's hard to let go, but you've got much better things to focus on than someone who consistently yanks your chain and makes you unhappy.