Gain back his trust. Simply be a friend. Hang out with him, talk to him, be friendly. Let him know that you're here for him 100%. I think that what things like this need most of all is time. Some people take awhile to fully trust someone, but if you don't give up on him I'm sure he will trust you in time. Just keep being patient, keep being a friend, and things just may get better. Good luck.