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.
