Pippi,
Sorry you are dealing with this! I left my STXAH and have two young sons 9 and 5. My five year old seems to be handling everything well. I think it is his personality. I have a few more issues with my nine year old. He takes the weight of the world on his shoulders and gets stressed and occasionally acts out.
What has helped me is getting him a councelor and one for myself. I talk to my son before his meetings and point out things that have happened during the week he may want to discuss with her. I ask him to ask for tools on better ways to handle feelings and situations. One of the best things for our relationship, is I see a councelor who also specializes in children and families. I ask for advice on how to channel my frustration which happens almost daily, when dealing with his Dad and the divorce. Also, on how best to react to and with him when tantrums or anger situation arise.
Understanding how much my reactions to him affect the moment has been really I opening. Small changes in my behavior have changed our relationship immensely.
4MyBoys