I completed my tax return for my son but forgot to add a w2

You will need to add both W-2's when filing on the same tax return. Please see the information below for instructions:

https://ttlc.intuit.com/community/entering-importing/help/where-do-i-add-or-enter-a-second-w-2/00/26519

You need to wait for the first return to either be accepted or rejected by the IRS. If rejected, you can then add the second W-2 and refile the return electronically. If it's accepted, then you need to wait for the refund to be issued, then prepare an amended return, print, sign and mail it in, and wait the 12 weeks for them to process it and mail you any additional refund you may be entitled to.

Please see the TurboTax FAQ below on amending your tax return:

https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-filed/00/25588