Hi, I'd like to help you with your question.
It sounds like you are trying to approach this problem the best you can, but your husband is not receptive to your feelings. That is where the problem is. Until your husband is willing to consider this an issue, it will remain a problem.
That does not mean that you cannot do anything about it. So far, you have taken very good steps in addressing the problem. You mentioned talking with your husband about how you feel, several times. You also went to individual counseling. How about counseling with your husband? You can either see a therapist together, or if you attend church, try your pastor. Pastors are often very good at marital issues. Either way, the point is since your husband is not listening to you, he may be willing to listen to others tell him this is a problem. Let your husband know that you feel this situation is affecting your marriage and that it is taking a toll on you and your wellbeing and you need outside intervention in this matter.
At your stepson's age, his relationship with you should be more on a friendship basis rather than parental. His father needs to be the one to maintain the parental relationship, including guidance and if his son is at home, setting rules and responsibilities. When he is at home, he should contribute to the household income, paying his share of the rent and bills. He should also be doing chores and picking up after himself. If he uses the car, he should bring it back with a full tank and contribute to the car repair and maintenance. Also, there needs to be a set time that he is given to move out. Although your husband can help, he should not be paying bills in full for his son.
You can set some rules yourself if your husband will not. These are not to be rules for your stepson, but rules that help you protect yourself.
If your stepson wants dinner, have him make it himself. If you want to make it, then assign him chores that help with dinner preparation. Set the table, chop vegetables, make a side dish etc.
Do not pick up his messes, let him do his own laundry etc. In other words, treat him as a roommate, not a child. If he leaves a mess somewhere that you have to deal with, dump it in his room. Otherwise, leave it where it is. It may be aggravating, but maybe once your husband sees the mess, he may realize his son needs to be more responsible.
Talk with your husband about setting guidelines. Tell him that you will no longer support his son and given that it is your home too, you need to have a say so about who lives in it and what goes on. Insist on a deadline for his son to move out. Keep your conversation level and not hostile. Be firm. Bring out a calender and set a date. Bring up the date frequently as a reminder. This may help your husband realize that his son needs to move on.
I hope this has helped,