If the situation with your son and your husband isn't resolved then your relationship with your son is affected as he will not want to visit you as long as your husband who he hates as you say is there. I have a similar situation.
Your son is most likely angry for being held to task, especially since your husband is not his father. Reaching out to mend the relationship is probably the best approach as your therapist says, maybe sessions with all three of you together might help. A good therapist can help lead you to the right decision. If your son can be reached and he decides what his consequences are for what he did maybe it will work better. But probably the first step is to mend the bridge between him and your husband.
If not, its broken. My adult son lived with me for a period of time and would lash out at me in anger, cursing saying f..... you. When my partner moved in who tried to blend in which was hard but he tried and heard my son cursing me he spoke to him nicely about it.....didn't work ....son cursed me out again ...my partner lost his temper and told him to get out. In some peoples world disrespecting your mom is not accepted....my son moved out (which turned out to be the best for him in the long run) but he has never forgiven my partner and as a result our relationship is broken. He will not come over, not visit and have any family events. He refuses to forgive for a man holding him accountable for his actions. That is something your son should realize, accountability for his actions.. Hopefully this will work out and have a positive outcome. Good luck.