Arob3837 - no one is saying it's acceptable for anyone, man or woman, to have such disregard for their child's feelings or wellbeing. Nor that it is acceptable to introduce them to a new partner so quickly.
What we are saying is that stopping contact altogether, denying your Son a relationship with his father is not the answer.
Believe me - I have been there. By 4 months after our split Ex was living with a new woman playing happy families every other weekend with my DS. 6 months later he had moved out and moved on. Just as DS was getting used to the situation. I cannot imagine how upsetting and confusing that would have been for DS. I tried to talk to his father and explain my feelings and get him to agree to access at his parents etc, but I didn't stop contact.
Imagine how upsetting and confusing it would be to have your Dad disappear from your life. For reasons you don't understand. Knowing your mother is angry at him. Surely that is just as damaging to a child, if not possibly more, than spending time with his new GF for a few hours?
Also it's not really a case of letting him do what he wants with your son. He is your ex's son too. And he has the same rights you do.
You have to accept you are both parents, but you both have different views and opinions on how to parent. The only thing you can do is work out how you can minimise the impact on your DS. Look to agree some conditions on contact. If anger is a problem perhaps look into what you might do to ensure your son's safety.
But please, think very hard about how you might be hurting your son by denying him his father completely.
As STIDW points out this could lead to a long and protracted court case or similar, and so could drag out your sons hurt, upset and confusion over everything for years to come.
I've seen this first hand - between my recent boyfriend and his ex. It was one of the reasons our relationship didn't work (nearly 5 years after they split up) and I am so glad I do not have to deal with the poison and vindictiveness that they both entered into. The person who came off worse was their poor son. A poor, confused, hurt, angry little 6 year old who was consistently used as pawn in adult conflict. Please don't let it come to that.