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.
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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.