Other than the obvious--transparency, non-defensiveness, giving you all passwords, counseling, etc--it really just takes TIME. Every day can be a building block in trust, every little thing.
You are going to be going up and down on the rollercoaster; one day you will be hopeful and the next you will be in despair. He will get frustrated at the slow pace of healing, 'cause this takes years. But if you both keep talking and keep working at it, it will get better.
You will never have 100 percent trust, because you now live in a world where he can cheat on you. But you definitely can get to a place where you believe he's a better bet trust-wise than another guy.
It sounds like you have a promising start.