[He trusts them, okay, that means he knows he can tell them what happened and know neither of them is going to run to Snow or Charming and tell them about it. Not if he doesn't want them to know.]
I know Porthos, he wouldn't tell anyone if I don't want him to. And Aramis... he wouldn't, either. [He might not have a stellar history of keeping secrets, but that's only when it comes to pretty ladies he's sleeping with, and Stiles is neither.]