After you file

Was his return efiled before yours?  Check his return and see if it is checked for  Someone can claim you on the Standard Deduction line in the top half of the 1040.  If he didn't claim Dependent then you will have to print and mail your return this year to claim him.  He will have to amend his return.   You don't have to wait for his return to be amended since you have to mail yours.