You just have to do it, not for him, but for yourself. If he is sincere, then he will earn it, if not you'll know that perhaps it's time to move on. If you continue on in the non-trusting paranoid mode for the rest of your life, you will go "nuts" and he will not want to be around you.
As far as the wedding, perhaps he is a little concerned with how either of you will react. He doesn't want your sitch to affect, or become more apparent at his friends wedding. At least he is taking his sister. Maybe, also this wedding will help he remember the good times.
Trust can be earned, but mainly it is something we give others, for ourself. It makes it easier to live in this world.
