Hi, I'd like to help you with your question.
This problem could either of two things. One, he made an honest mistake. It shouldn't happen but it does. You can let him know that this upset you and that you find it hurtful.
The other option is that he is still carrying on the relationship. Although that is not something you want to think about, the fact that the name of the ex was on his mind says there might be something there. You can try asking him straight out about it and see if he will talk about it. However, the fact that he will not attend therapy says he does not feel there is a problem. That is a dangerous sign since you feel that there is an issue.This affects your ability to trust that you are first in the relationship.
Reintroduce the idea of counseling with him. Let him know that you were bothered by being called the wrong name and that you feel things are unresolved. If he refuses, continue to go alone but let your counselor know what happened. Also, you may want to reevaluate your marriage. If he continues to deny problems and is cheating, it will destroy the trust you have built so far and undermine any progress you try to make.
If you attend church, talk with your pastor. Pastors are often great resources for marital problems and can give you support. Also, talk with trusted friends and family about your situation. Help yourself feel better by pampering yourself and caring for yourself. When infidelity happens in a marriage, your self esteem often takes a hit. Taking care of yourself and gaining support will help you feel stronger and better able to make a decision about what to do.
A few books that may help you are Infidelity: A Survival Guide by Don-David Lusterman
and Not "Just Friends": Rebuilding Trust and Recovering Your Sanity After Infidelity by Shirley P. Glass and Jean Coppock Staeheli. You can find these at Amazon.com or your local library may have them for you.
I hope this has helped you,