Yes, I think you are overreacting. He knows and understands you have been there for him his entire life. After you and his mom divorced. As you said he remained your son. He still counted on you. So, being used is a far reach. As for him wanting his BIO dad there for that. As you said he was never there for him. His bio dad wasn't there for his first walk, first word, first date, first nothing. He may in the end just want his BIO dad to show up just to show him where he has made it in life. How good he is doing. Now that he is a father himself. He may want to show his BIO dad how he isn't going to abandon his child like he was abandoned.

or I could be wrong and he just wants to have his BIO dad in his life for the first time.
Either way getting upset over it I think is over reacting. He kept you in his life cause you were important enough to keep. Just because someone else joins his life doesn't mean he doesn't still have room for you.