What do I do? I am scared if I cut off communication completely he will go off the deep end.
are you sure that's the reason? if he is telling the truth - then it was after you LEFT that he made the decision to do something about his drinking. NOT while you were there. and if he is half serious about sobriety, he can do that all just fine WITHOUT you.
it's only been a month. you've just landed back home. you hardly have your bearings. you traveled a great distance to get away from him and to return to your support system.
go no contact. let him have his life and let him go. free yourself. there is so much more to living than this.