Thought you might enjoy reading this summary of a parenting consultation I did last night for a divorced dad whose six year old son was having problems with anxiety. Seems to me that some of this advice could be relevant for fathers and sons without issues as well ....
Your son might prefer to connect with you while doing a physical activity side by side rather than tell you his feelings; try shooting hoops or playing a video game together rather than wanting him to sit and talk. Watch for things you and him do together that you both find relaxing and fun - and plan to do more of that.
What boys want more than anything from their dads is approval. He hopes you see him as strong and capable and smart. You are his hero, so he takes your opinion of him very seriously. Don't take that responsibility lightly.
Remember that whatever you focus on in him, he will want to do more of. So focus on his resilience and strength rather than his temporary feelings of anxiety.
Decide carefully what messages you give him now, because your voice is so important to him that he will carry it in his head forever. One day, he will share it with his own son.
When he gets anxious, the best thing you can do for him is to keep yourself calm. If you start to feel upset or pressured to make him feel better, take a few deep breaths or a drink of water or a bathroom break to settle yourself down before you try to be there for him.
After you are calm, then just be there with him. You don't have to fix the source of his anxiety - sometimes he won't even know what triggered it. Just be there, sort of like a big strong calm rock in a stormy sea.
Being strong and calm yourself sets an example for him that you are not worried about him, that his anxiety will pass, and that you are not going to leave him all alone to cope with it.When you stay calm, it's almost like you are a life raft on that stormy sea, and he can climb in with you and feel safe. That's what every kid needs from their dad - shelter in the storm, and strength to borrow when they can't find their own.
And as you mentioned, your anger at his mom could easily pollute your relationship with him if you let it. So find some other way to deal with your thoughts about her. Challenge yourself to never speak negatively of her when you are with your son.
She has no power over your relationship with him - that's all up to you. Be the best dad you can be, and leave her out of that equation.