Go to the following link for instructions to amend a 2018 tax return that was filed with TurboTax.
An amended return has to be filed on a special form, Form 1040X. It cannot be e-filed. Your son will have to print it, sign it, and file it by mail. It takes the IRS up to 16 weeks to process an amended return.
He should not file the amended return until his original return has been accepted and he has received the original refund.
However, even after your son files his amended return you will not be able to e-file your return this year. You will have to print your tax return, sign it, and file it by mail.
Depending on your son's income, 1040X line 2 might change, since there are special rules for the standard deduction for someone who can be claimed as a dependent. That would result in changes in some of the lines below that. It sounds like you are filling out Form 1040X by hand instead of using TurboTax. It would be safer to use TurboTax, to make sure that the standard deduction is handled correctly.