You shouldn't have to beg your husband to have your back or to stand up to his mother for the sake of your and his own son's health. That is a lonely place to be.

Have you mentioned this to the counsellor? Your husband needs to be accountable for his actions, or lack of follow-through. You can't work at the marriage by yourself, he needs to meet you halfway, or at least part of the way. Otherwise, this won't work.

Is your husband having individual therapy? He seems to be cowed by his mother and unable to stand up to her. IF your husband is bringing past baggage into this relationship, this baggage (e.g. mother's hold on him) will affect the marriage until it is dealt with.