As Mike9241 stated, you can file amended returns and claim your dependent if you were the custodial parent each year. You were the custodial parent if the child spent more than half the year with you. As long as the child was living with you, you qualify to claim the dependent and take all the child related tax credits. Even if you had given him the Form 8332, as a noncustodial parent, all he can claim is the exemption ($0) and the Child Tax Credit (and he needs the form from you to even do that). The ability to qualify for Earned Income Credit, Head of Household, and the Dependent Care Credit, always stays with the custodial parent and can't be waived, shared, or transferred.
What Happens When Both Parents Claim a Child on a Tax Return? If you file amended returns to claim the child, the IRS will intervene and if he was not the custodial parent, he will likely be required to amend his returns and pay back any credits he did not qualify for. The IRS is not going to allow him dependent credits for a child that did not live with him for more than 6 months. If he answered the dependent questions honestly, and answered the child only lived with him for one third of the year, he would not have received any credits or exemptions.
How to amend (change or correct) a return you've already filed
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