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,