How can I prove this to him?

I think he's saying that as a way to put a rift in your relationship. This sabotage is either because he may want a future way out or is testing your resolve. Either way, it's not a good sign. You can't "prove" you can be trusted with words, only actions. If you want this to work out, don't talk to him about previous trust issues with other guys or your just setting yourself up for him to meet your expectations. Let your past stay in the past. Act today how you want to be treated tomorrow. Tell him (or the next guy) that you DO trust guys (you may have to lie about it at first) and you'll be starting a new pattern that he will be expected to follow.