To play devil's advocate here, if he needed a rest why did you let him go next door? Especially if you had read his emails and knew he was going to betray your trust. Did he know the reasons why you didn't want him to go to the girls house? And does he agree that at almost 13 he is still to young to go out after dark because for many kids they would find that unfair. It sounds a little like you were trapping him into disobeying you and TBH there is a betrayal of trust on both sides here. Was he aware you were reading his messages or did he think they were private? I totally agree that he needs to understand he has broken his trust but equally if you are not trusting him with age appropriate play (such as walking out after dark) and reading his private messages then it's not that far flung a concept to me that he will start betraying your trust back.