This little exchange is ironic. Nick says he can only trust himself. What he doesn't realize is that "Uncle Ambrose" is he himself from the future. So, yeah, he can trust Uncle Ambrose, because they're the same person.

Caleb is right when he says this to Nick. Although it again sounds like they're talking about being business partners instead of friends, they're one and the same when you're side by side facing off against zombies.