You could try a child psychologist. Some of them are free. They will simply talk to him about his feelings. Separation is such a big thing for kids to go through at any age. You know that you and your ex are still friends etc, but have you sat down and explained it all to him that you both still love him and will stay friends etc. sometimes he may just need to hear that. He's probably feeling hurt and confused. My sons going through the same thing. He's six. Good luck .