How do you gain a persons trust back?

Be consistent in everything you do, and be completely honest when you answer his questions, no matter how tough they are. Don't be misleading in any way, no matter what. You'll just have to give it time - I know that sucks, but trust is something that takes a long time to build and just a few seconds to tear down. If he's really worth it, put in the time and invest in your relationship. Also, make a heartfelt apology once. Don't keep saying it over and over. You might write him a letter (not an email, a letter) explaining your desire to be friends and your willingness to work to regain his trust.

I suggest giving him his space because if you come off too strong or persistent, it may push him away. Let him know that you're there for him when he's ready and leave it up to him. Good luck!

Well depends on what happened. Earning a trust back is probably one of the hardest thing to do! The best thing here is to just hang in there. Whenever you hang out with him, just be honest and open and feel like you aren't to blame and things will be okay. Building the "long lost trust" will obviously take time and as you guys hang out and are back to being friends and such that trust will eventually form.

The big pointer here is that. All you can do is try your best to be you and not repeat what ever happened before. Trying a new hike into the new relationship as friends and bonding the trust.

Good luck.