Sounds like a difficult situation. Sounds like the two of them just don't get along, and because you weren't there the day of their fight, you'll NEVER truly know what exactly happened.
You talk about leaving your husband, is that because of your son, or are you having that many problems in the marriage? I agree with your therapist, that of you divorce your husband to get your son back in your life, you may end up disappointed. You may find that your son will hold his ground.
Is your son married? Independent?
There are lots of adult children you don't like their parent's new spouse, but this sounds a bit more complicated than that, it sounds like he feels that your husband did something to him, and you're taking your husband's side? You cannot make your son come around, even IF you divorce your husband.
My advice to you would be to perhaps write him a letter, or send him an email one last time. Just be brief, don't bring up all the drama with him and your husband. Simply tell him that you love him, you're so sorry things ended up being such a mess, that you respect his feelings toward your spouse, and don't expect him to have any kind of relationship with him, but that you would like a relationship with him, independent of your husband. That's all you can really do....open your door to him....and put the ball in his court.
Good luck to you...hope things get better.