If another taxpayer is entitled to claim you as a dependent, you cannot take an exemption for yourself even if the other taxpayer doesn't actually claim you as a dependent. So if under the IRS rules, he is your eligible dependent, he can not claim himself. He may still have to file a tax return, he just then must answer that someone else can claim me....
Do note the IRS will only allow 1 return e-filed for a dependent. If he has already filed and claimed himself, you can only file by mail and he will need to amend his return. If you both tried to claim him, they will audit the returns and normally decisions are with the parent, there are some students who can claim themselves such as military.
