Pretty much the same as every other character development; weak to strong, enemies to lovers, immature to mature, and more. Since in your case it's from distrustful of everyone to trusting, all you simply have to do is challenge your MC's belief: "I can't and shouldn't trust anyone, otherwise it would bite me in the ass."
Have your MC be surrounded by people who sincerely are good or wants to help him, but the MC keeps rejecting them out of fear of betrayal. Slowly but surely, he's learning to trust again because of the people around him; make him encounter situations that pretty much forces him to choose to confront his fears or run away (which he confronts since it's a positive character development), but he still can't get past the final hurdle, not until near the end of the volume or book, where he realizes that he can't defeat the external antagonist alone so he asks for help by the people earlier (in here, make sure the MC's stakes are very high, like possibly losing someone he subconsciously cares about, so that the impact of him asking for help, which is a sign of trust, is strong).
Watch "Me, Earl, and the Dying Girl" if you can't get a flash of inspiration. Although there's no betrayal, the movie is about learning to trust and friendship, which would probably help with your story.