What do you do when your boyfriend likes another girl?

Help well ok so I have been going out with this one guy and it was all going good you until his ex started to talk to him agaiin ma boyfriend and his ex sent out for bout a year and something months and the were each others firsts in everything and I know that that has bonded them in a way . They broke up like 7 or 8 months ago because of some other guy getting in between and well he moved on by asking me and the problem is that what do I do when he just told me that he may have feelings for her ? I gave him a week to try and sort out his feeling but I don’t know why im feeling really shity and I cant seem to trust him especially because I dont want to loose him and I know that she has a greater advantage to get him back because they have the same friends so I need help on what should I do.. I really like this guy and he tells me that he is really scared because he has never had this happen to him and he is the type of guy that knows what he wants so this happening to him is rare, and he told me that he dont what to make the wrong choice and that he is scared that if he dump me or take a break and he realizes he misses me I may not be there for him but then again he still thinks about the other girl so please any advise because im feeling really bad and I really don’t want to loose this guy at all!!!