Welcome Back to JustAnswer! I don't think he should have stopped counseling just because things felt better, I think your husband started getting a false sense of comfort and felt he didn't need counseling anymore because he was starting to feel better and I would suggest to you that he do individual counseling so he can figure out on his own why he continues to think about this ex girlfriend. The counselor needs to find out just why he continues to think about her and says he wants to stay in the marriage something in his subconscious keeps him thinking about her. It could be the fact that he may not have had closure with the relationship, sometimes in relationships when a couple breaks up and one of them don't understand why and doesn't get answer for the reasons why and they think about what happened and what they did wrong and because they go unanswered they are left still wondering over the years and because they didn't have that closure they never quite get over that relationship this could be the case with your husband and the only way he will find out just what it is he has to start individual counseling and why I suggest individual counseling is when you went with him he may have been holding back talking about all of his feelings because he didn't want to upset you.
If you want your husband to get better and get over this woman then you may have to allow him to do this on his own and trust that this is going to make your marriage and your relationship with your husband much stronger. If he can be more honest and open about his feelings then the counseling can help him figure out why he feels this way and what steps he needs to take to get over this past relationship. It may include contacting the ex again and asking questions. Are you will to allow him that if it gives him closure? That may be a question will have to answer in the future as part of his healing process.
Yes you're right he needs closure but also needs to talk to a counselor about his feelings and what he needs to say to her to make things better, it may give him courage to do what he has to do to save his marriage. Just try looking at the big picture if he gets closure she will no longer play a role in your marriage. Think positive and pray about it.