She's obviously angry with you at the moment, but is she the type to cool down when she's thought about it. How old is your son? Is he old enough to pick up on anything going on between you and your ex.
Is there anybody you can get to speak to her about it. A family member or friend that can act as a go-between and try to make her understand that she's only hurting your child. Unless you are a mad axe murderer of course and then she'd be well within her rights to deny access.
Failing the softly softly approach you'll have to seek legal advice. You can usually get a free interview with a solicitor if you go through the Citizens Advice Bureau. Good luck!