Alright. So it sounds like his distrust of you makes it hard to communicate. Not surprising. But especially when it comes to other people. Hmmm.

Something to try is remind him the conversation at hand isn't about you and him. It's about him and MK. And you, Mk's mentor and mate, and him, his other mate, need to talk about this. And that you need him to be honest with you for Mk's safe. He doesn't have to trust you or forgive you but Mk deserves it. Because you both can't help him if you aren't honest with one another.