Boyfriend has PTSD

Alicia

My boyfriend has PTSD as a result of me cheating on him twice right at the beginning of our relationship and then telling him about it. I tell him I regret it but he still has 'bad moods' where he imagines me cheating on him and feels like he wants to break up with me. We have been dating for 9 months now and he still can't get over it. We're both only 15 years old and he doesn't think he need professional help. What can I do to help him?