"ME AND MY SHADOW!" That's exactly true! He'll be trying his very best to be in plain sight when you are around and be as good as he can be. He'll probably try to please you and be a real good boy. Anyone can make one mistake and although I don't blame you for mistrusting him right now (he has broken that trust with you) he does deserve another chance. You just never know if you'd be in a situation of cheating as he was. Most of us never think this could happen to us, but given the right circumstances and the maturity level of the person it's possible.
If he is sneaking phone calls, going off to another room while talking on the phone, working a lot of late nights or not turning up at home until late then I would say you have much to be concerned about.