Well, I think the underlying issue here does not seem to be whether it's a fetish or an addiction, but rather that whatever it is, he seems to feel a lot of shame about it.
If he deemed his actions socially acceptable, and acceptable amongst his peers and loved ones, he probably would not feel as much shame, which showcases the relative nature present in some moral issues.
He feels shame, which may take root in other issues as well. And that shame seems to create a pattern of negativity in his thought process, damaging his self-esteem.
I felt pleased to read that you are no longer together, but still you ask here for direction. That shows caring, but that perhaps you became aware of the destructive nature of the place he finds himself in at the moment, and how that could affect you.
I am one who believes in getting help when needed, whether from professionals or friends. I am also a believer in our ability to heal ourselves emotionally. In the end, all a professional or friend can do is guide you through the process, which no one can do for you. In his case, it sounds like he's in need of guidance.
He will have to possess a great deal of honesty, will power and acceptance to regain his foothold in the land of the stable of mind and high of self-esteem; but with some care and time I believe he can get there.
Also, I just want to bring a side, but still relevant, point to the discussion.
Messaging others in an intimate way, flirting, and even sexual relations with others while in a committed relationship are not necessarily wrong. Unless it breaks boundaries and agreements established by both parties within the relationship. I have met women who felt fine with their mates displaying interest in others, and I have known those who would stay a mile away from any such men. It all depends on what you want in your life.
Perhaps he will find that there may be a healthy side to his sexual wishes- after examining his shame and negative feelings. Perhaps not.
If he ever does, though, and decides that it is relevant and important to his life that he continue certain behaviors, there is bound to be people who will accept it.
However, as a final point... it should be noted that any sexual deviance is likely to decrease the available pool of potential mates out there, making it harder - though not impossible- to find compatible partners.
Not a fetish. Like mami said, it's an addiction and many types of sexually compulsive behavior can become addictions. Who knows what he said to the therapist years ago- he may have minimized it for fear that he would officially be labeled a freak, but he needs to get help. Support groups can help, but he needs to find a therapist who works with sex addicts. Every state (if you're in the U.S.) has counselors who are licensed to work with sex offenders (I know he's doesn't have that legal label, but we want to keep it that way), and that's what he needs to find. Once he does that, he needs to go into it being completely honest. If he holds back any information about his habits or sexual history, that will only hinder his treatment. Good luck.
If he honestly wants to stop he needs help. Otherwise he won't kick the addiction. If he goes into therapy thinking it won't help then it won't. He has to give it the benefit of the doubt and try to make it work. I have to honestly say, however, that from what you are telling me I got the feeling he was just saying what he is so you stick around and feel sorry for him. It seems like he wants you to verify that he's not a freak, creep and that he is worthy of a girls love. That way it gives him "permission" to keep doing what he's doing and not have to feel guilty about it. I hope it's not true but it's just a gut feeling I got after reading your post.
He needs to see a different doctor and make it a point that THIS is what he wants to discuss and THIS is what he needs help with first and foremost.
If he's depressed about it then it seems like more of a sexual addiction. He can seek help for that, through therapy or support groups. He must be treated like an alcoholic and follow the steps to recovery. He needs to want to get help for himself. You can advise him to seek counseling but he won't change for you. Now you have to think if this man is worth going through all of this with, that is completely up to you and what you are willing to tolerate. I hope he is serious about how he's feeling and it's not just a way to keep you sticking around while he messes around. Good luck.
I think it may go beyond a fetish as it seems to have a negative effect on his life and would cause him to have trouble maintain a relationship.