My opinion is that if you don't trust him, don't tell him. Even it would make you vulnerable, don't let him use it. Wouldn't he just go and tell your new boss? Have you considered talking to your new boss and letting him know that you have made positive changes in your life (not saying anything specific) and that you appreciate your new position and opportunity? In other words, put a positive spin on it to your new bosses, without specifying what the problem was.