Hello If your child still lived with you, was under 19and you provided more than half of his support he needs to file a 1040X to remove his persona;l exemption from his return. You will not be allowed to e-file so you will need to mail in your return and claim him as a dependent. Next year if he is not a full time student, lives on his own and makes more than the personal exemption amount (2011 is $3700) then he can claim himself . Even if someone else can claim you as a dependent and does not, a person is not legally allowed to claim their own personal exemption.
I sincerely hope this information is helpful and thank you in advance for clicking ACCEPT.
Please advise your son to amend his return by filing Form 1040X. You cannot have him as a dependent and he claim his own personal exemption too.