Most Helpful Guy
Tell him what you just did and appoligize. You also want to gush over him a bit to let him know how much you appreciate the way he treats you. You showed a hint of jealousy which is understandable and this will tell you a lot about him. Considering he focused on you after you started dating, I sounds like you have a keeper without jealousy issues also.
Most Helpful Girl
I would just explain to him that you did go through his phone and you apologize for invading his privacy. You should have had enough faith in him to know that he isn't the type of guy to do anything that'd hurt you and that it won't happen again. Maybe that'll make you feel better to know that you were honest and upfront with him while assuming he wasn't.