Did he live with you at least half of the year last year? which is the main factor for claiming someone as a dependent.
This is a great page that talks about the steps for what to do in your case
http://taxes.about.com/b/2007/03/21/what-if-someone-else-claimed-your- child-as-a-dependent.htm
if you meet all the situations to claim him, it will be an easy win for you, and he will have to modify his tax return for the IRS.
downside, it will just take longer to get your return.



