You are correct to think that this is wrong.
Your husband should have taken the advice of the marriage counselor.
If he is earning that much money, he needs to be living on his own. He is an adult. He is taking advantage of the situation he is in right now.
You and your husband could spend a day helping him look for an apartment, the son might even get excited about the idea.
If your husband is against having him move out, he at least needs to be learning some kind of responsibility by paying some rent and buying his own food.
BotXXXXX XXXXXne is that something needs to happen. Only you and your husband can decide but I would opt for having him get his own place.
Thanks and good luck to you!!