Where Can I amend my 17 year old's 1040 to check the Someone Can Claim: You as a Dependent box so that I (his parent) can claim him as a dependent?

His return was filed in March, accepted and his refund already received. I am now trying to file my taxes and it's getting rejected because DependentSSN is listed on his return and that box isn't checked. I can't find where to update it on an amended return.