The IRS doesn't care about his court order, unless it is dated before 2009. The IRS goes by it's own rules and will award the dependency to the custodial parent (you), if both parents try to claim the child. His only remedy against you is to take you back to court for sanctions.
"The IRS will not award a dependent claim of the non-custodial parent without a signed form 8332 from the custodial parent. Everything else is a matter between you, your ex, and the judge" (from previous reply above).
If someone else claimed your child inappropriately, and if they file first, your return will be rejected if e-filed. You would then need to file a return on paper, claiming the child as appropriate. The IRS will process your return and send you your refund, in the normal time. Shortly (up to a year) thereafter, you'll receive a letter from the IRS, stating that your child was claimed on another return. It will tell you that if you made a mistake to file an amended return and if you didn't make a mistake to do nothing. The other party will get the same letter you did. If one of you doesn't file an amended return, unclaiming the child, the next letter, from the IRS, will require you to provide proof. Be sure to reply in a timely manner.
Winner gets the tax benefits; loser gets to pay the IRS back with penalties and interest. The custodial parent almost always wins.
