Your son is an adult. He should act maturely and push the past aside. There is nothing he can do to change the past. However, you can shape the future.
Overall, yes, your twenty-three-year-old is overreacting.
I hope this helps!

No.

Its understandable but it also shows immaturity. He is a grown man, and you cant change the past.

Well, they have the right to be upset because we are all born free. Also, when a parent treats a certain child differently than the others, there is a good chance they might not like that. I have witnessed such events happen with other people I know and they do not just get over being upset instantly. It might take some time for your twenty-three year old to adjust and move on. You have to understand why your child feels this way though, you cannot act like you have done nothing to cause this. I would not say that they are being immature, they are a person and human beings have feelings. If you think they are being immature and you are upset about that then, you are the immature one. You should confront your twenty-three year old, tell them that you never intended for this to happen and you love them all equally. Apologize for what has occurred, be the bigger person. Your child will eventually let go of this anger because, you are their mother and they love you. I hope everything works out and your family finds true peace and happiness. Have a great new year.

Seems like you were playing favorites with the other kids and being more harsh on him. He has a right to be upset, he is not overreacting. When you treat the other kids differently don't expect for him to feel the same toward you as they do. He's built up resentment toward you and you should really take a step back and see what you have done to him. In his eyes he was the black sheep.