Hi, Free lancer. Did this boyfriend know the ex? I don't understand about his thinking about you and the ex. If he didn't know the guy then it is up to him to fantasize or not about whatever he "wants". I don't know that he can be "helped" with that as it is all in his thoughts and head. It isn't about "you" at all so there's nothing much you can do about it.

I think for awhile he needs to deliberately think about something else more "interesting" or see a counselor to figure out why he is concentrating on something in your past instead of his own life.