As far as the IRS is concerned, the custodial parent is the one who can claim a child as a dependent. If your ex claims the child and files first, you can still claim your child on your tax return, but your e-file will be rejected. Just print, sign and mail the return instead. It will be handled differently by the IRS. Eventually, both parents will receive letters from the IRS to sort out who should have claimed the child. The one who should NOT have claimed the child will be subject to repayment of refundable credits and penalties.
There is nothing you can do to prevent him from doing something wrong ... your ONLY option is to also file a return claiming the child and let the IRS figure it out. If he files prior to you then you will need to mail in the return to assert your claim on the child. If you did not do this for last year then amend the return now and mail it in.
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. The non-custodial parent can only claim the child as a dependent if the custodial parent gives permission (on form 8332) or if it's spelled out in a pre 2009 divorce decree.
For more info, see http://taxes.about.com/od/dependents/qt/Dependents-Audits.htm
But, there is a way to split the tax benefits. For future negotiations with the other parent (and maybe even for this year) the following info may be of use:
There is a special rule in the case of divorced & separated (including never married) parents. When the non-custodial parent is claiming the child as a dependent/exemption/child tax credit; the custodial parent** is still allowed to claim the same child for Earned Income Credit, Head of Household filing status, and day care credit. This "splitting of the child" is not available to parents who lived together at any time during the last 6 months of the year; then only one of you can claim the child for any tax reasons. The tax benefits may not be split in any other manner.
Note in particular that the non-custodial parent can never claim the Earned Income Credit, Head of Household filing status or the day care credit, based on that child, even when the custodial parent has released the exemption to him.
So, it's good idea to let the other parent know that you will be claiming those items, as many first time divorced parents are not aware of this rule and may try to claim those items, which will cause the IRS to send out letters.
Scroll down to "Children of divorced or separated parents (or parents who live apart)"
** The IRS goes by physical custody, not legal custody, for who is the custodial parent. Furthermore, for tax purposes, there is no such thing as joint custody, regardless of what your legal agreement says. The requirement, to be custodial parent, is that the child live with you MORE than 50% of the time. One of you has to be the custodial parent and the other the non-custodial parent. The IRS goes by physical custody, not legal custody.
You cannot prevent someone from claiming a dependent on their tax return.
You should file your tax return and claim your son as a dependent. If the IRS receives tax returns with the same Social Security number for the dependent on the returns the IRS will investigate. They will send all parties a notice to amend their tax return if the dependent was incorrectly claimed on the tax return. If no party amends their return then the IRS will send another notice to all parties requiring proof that they were eligible to claim the dependent. The losing party must repay any tax refund based on claiming the dependent along with paying penalties and interest.