if someone already claimed him all that can be done is to paper file.
there should not be a problem in e-filing his return as long as it's mark the he an be claimed as a dependent. if this is not true then his return will also have to be paper filed
what will happen is that the IRS will eventually send you and the other claimant a letter asking for proof your entitle to claim your son. the letter will contain items you can submit as proof. the one with the better proof gets him as a dependent.
