When the two of you are out together. Tell him that your battery on your phone is dead and ask to borrow his for an important phone call. You can check his call history and text messages from there, but I don't know how you would figure out what his code is.

Another option would be to simply confront him the next time his phone rings and he doesn't answer it. Just say, "who's that?" or "why don't you answer that?" You could ask him why he's being so secretive from that point...