How do you rebuild trust?

About 9-10 months into our relationship I discovered my boyfriend had nudes (of himself) on his phone from various months early in our relationship. He's never sent me any. I asked him about them and basically got a run around as to why he had them. He denied them until I showed him the proof. He had an account on a website where you basically post nudes and you can talk and hook up with people. He denied that til I showed him the proof. It was about 2 months in last time he was on his account. It's been over 2 years but some of the trust is still missing. I feel like he is hiding something from me but I also feel like it's still issues from the beginning of our relationship. I want to trust him but don't know how to.