Hi, I'd like to help you with your question.
Some people have difficulty accepting the truth because either it doesn't fit what they feel or they are not able to deal well with what they are being told. And to accept responsibility means they have to make amends and most people don't like how that makes them feel. By not accepting the truth, they avoid feeling bad especially if they already have low self esteem.
If you are at odds with your son's father, it may also be a power struggle. If he would accept what you say and respond to it, then he would lose power in the relationship. So instead, he chooses to ignore you or not accept what you say. That keeps him at a level with you or makes him feel he is higher than you since he is in control ("you can't make me respond" kind of power). The more upset you get, the better and more in power he feels.
I hope this helps,
I can't think of any other reasoning that could be behind such behavior but since I do not know your son's father, he could have his own personal issues I am not aware of. I just gave you the reason's I would consider first if I had someone like your son's father in therapy.
Choosing how to respond is based on what you want out of the interaction. If you are trying to maintain an emotional based relationship with your son's father, then it is more complicated. You are expecting a reaction and if he is like I described before, he is not going to respond if he wants to hold onto the upper hand.
If you are ok with a more neutral relationship, then you can decide to communicate only the most necessary information he needs to know. Keep your voice level and with the least amount of emotion possible (as to not provoke a response) and keep the communication on an as needed basis. Otherwise, do not allow yourself to be provoked in any way and try to see the situation in a logical manner rather than an emotional one, necessary in order to raise your child the best way possible. If you keep your son's well being as the sole focus, then it should help you accomplish this and make it easier to communicate.