The fact that he is doing the same or old behavior after you caught him means that he probably wont stop. He is moping around to make you feel sorry for him when it was him who was doing the behavior. You should not be feeling guilty at all. If you really want to move on then break it off with him. If you want to stay, then you have to decide if this behavior is something you can live with. Chances are, he is not going to change until HE wants to change!
The key here I believe is that in a relationship you have to accept the person for who they are, good and bad. I believe that far too many people get together thinking they can change the things that bug them about the other person once they are together. It just doesn't work and only builds up resentments. Either learn to live with the behavior or truely move on. All men masturbate, some several times a day. It really has nothing to do with you. It's a guy thing. If you are really concerned, ask to do it for him or maybe ask if he would like to masturbate for you and you for him. As long as he is not doing it with another woman then ask to be a part of it. All guys do it, besides, maybe he is fantasizing about you when he does it! :)