Child tax exemptions problems. Please help!!!?

Being unable to find a lawyer I ended up with a divorce where it says that I am a custodial parent but my ex will claim our child as dependent every year. No odds or evens. We divorced since December 2008. He does not let me even claim our son as a qualifying child who is not my dependent threatening to take me to court or pay what I'll get from tax return to him. This year he demands me to sign a form 8332 saying that I have to follow the divorce decree. But he, himself is breaking these rules constantly. Could anyone tell me please, if I refuse to sign this form, would he be able to claim our son as his dependent anyway and what should I do in this case? We'll he take me to court for refusing? Also, if divorce decree papers are higher than IRS rules, how can I change this? I appreciate any help and tips. Thank you