Well, I read a little of the advice you have gotten and I have to say that it is totally up to you on how you proceed. Personally, before just letting it go, I would try to talk to your son one more time.
I would start by telling him that families are made by two people coming together to make a new family usually through marriage. That while married these two people are choosing to be related to both their own family and those of their spouse. That when they have children, the children are related to all of those people related to both their mother and their father. I would tell him that both the mom and dad will always be related to the children but when they divorce, they make a new agreement that they are no longer related to each other's families just to their children. I would go on to tell him that although you and your ex will always be part of your son's family, your ex is no longer related to your mother or sister or etc. However, since he married your son's stepmom, he is now part of her family as well. Since they have had a son, they have created a new family too. I would tell him that he is a very lucky young man because is he is now part of two families. While he gets to be part of both because of his dad, you and your side of his family are technically not part of that new family. I would tell him that this might be confusing because even though you are not part of this new family, you still care about them and are grateful that they are part of your son's life and family. I would tell him that you are technically not related to the baby but are very happy that he has a new little brother and would love to hear all about him. If he is still a little confused, perhaps you could enlist the help of your ex in addressing the issue. Tell your ex you want your son to understand that you are not the babies step-mom but that you still care and are happy for both your ex and his new family and your son. Perhaps the two of you together can help your son to understand the new relationships.
If, however, you can't seem to help him to understand the many new relationships in his life and how his new little brother fits in with the rest of his family, I would let it go for another day or year.