nothing to do to stop other person from claiming, but he would be your qualifying child entitling you to claim him and file as head of household. if he claims and files first, you will not be able to e-file. you'll have to mail in your returns.
if both claim, each of you will get a notice from the IRS asking for you to amend. you should ignore it. if he doesn't amend ther's a second notice asking for you to prove it. the one with the better proof in th eyes of the IRS will get him and the other will get a bill.
