For some people, the existence of the child from a former relationship is a constant reminder that their partner once had another lover and another life. They would like to erase that past but can’t due to the presence of the child. On a more primitive level, male animals often won’t accept the offspring of other males and drive them out of the herd in an instinctive effort to only pass on their own bloodline.
People can and do get past that primitive behavior and open their hearts and minds to children who are not their own biological kids. All this is a long way around to saying that I think that the problem here is that your husband hasn’t adopted your son. He may not be aware of it, but his behavior seems to show that he’s trying to drive him from the herd. Of course your son knows it and is hurt by it.
Your husband needs help understanding the need for an “adoption” to take place. I’m not talking about a legal adoption. Your son does have an involved biological dad. But there is room for the boy to have two men in his life who care about him, teach him, and help him develop into all he can be.
Rather than be angry at your husband, try to have a conversation with him about what it is that gets in his way of being a positive role model and caring adult in your son’s life. Is there something you or your ex is doing that is an obstacle? Has he really thought about his role as being important to the boy’s self-esteem and growth? Has he embraced the fact that the boy is related to him by blood in that he is his sons’ brother?
If that conversation becomes too difficult, I hope you will try some couples counseling before you visit a lawyer. This is not an uncommon problem. It can usually be addressed so that all relationships in the family improve.
I wish you well.