Even though you might not connect his social withdrawal with you and your husband's separation (because he withdrew socially before the actual separation), my guess is that he observed disturbing exchanges between you and your husband long before there was an "official" separation. I think that he was probably confused, overwhelmed and scared about what would become of his family long before your separation. His devoting all his spare time to being with you or your husband might be the only way he feels he can hang on to a sense of family with you. He might even believe that he was one of the reasons that you separated.
Kids often believe they are the cause of their parent's separation and divorce, even when their parents tell them that this is not true. He also may be spending so much time with you because he needs the reassurance that you still love him and want to be with him. He might think that he could bring you back together again.
I strongly recommend that the three of you see an experienced family therapist. It may turn out that the therapist wants to see your son on an individual basis, combining his individual sessions with sessions where she sees all three of you together and you and your husband as a couple. Your son is being cheated out of the social growth and development that he needs. I know that you will get your family the professional help that you need.