Just explain to your son that his Dad is really busy. It's not that Dad doesn't love him, it's just that grown ups have responsiblities. Explain that his father still loves him and will spend as much time as he can. In reality most fathers, even the nondivorced ones don't get to spend as much time with their kids as they would like. It's not because they don't love them, it's just they are so busy providing for their families, which they view as showing their love. Take my husband for example, he works 2 full time jobs so that I can be home with our kids and go back to school. So he doesn't get to see us very much at all. It's not because he doesn't love us, on the contrary it's because he love us. I don't know the situation with your ex, but I'd guess deep down you know the ex really does love his son, he's just got other things going on right now also. If your son is old enough you could possibly help him with ways to talk to his dad about how the situation makes him feel and then maybe dad will take it better. He's more likely to listen if it comes directly from your son rather than you. Good luck!