I am so sorry for what brings you here. You say he takes Xanax on occasion. This means to me he is buying it illegally, is that correct?? I don't think that would be ok with me. My X husband is addicted to Xanax that is actually prescribed to him. Problem is he mixes it with booze. It can cause dangerous, blackout issues.
He has told you this is what he is going to do, believe him. Now you need to put the focus on what is best for you and your children.