@fortmacchick,
Quote: I trusted him when he said it was over and he made a huge mistake.
No, you did not trust him or you would not have been checking out his FB messages. (Not judging you here, simply stating a fact.)
That depends. If I were your boyfriend and I claimed that I loved you, then all it would take from you would be ask me not to contact this girl because of the past stuff that went on, and I would not do so. It does not sound as though he had much of a relationship with her, other than the trading of nude pics, unless there was more to it than you are saying. So in my opinion, it should be easy for him to stop contact with her. (I assume they were not really close friends for years, or some such thing.) Since he seems to have chosen not to break contact with her, then you are certainly justified to question whether this guy cares enough about you to have a long term relationship with him. Or maybe you just need to sit him down again and let him know in no uncertain terms that his continuing to contact this particular woman is going to be a major problem and he needs to choose between contacting her and you.
Good luck to you.