He will have to mail his Single return this year even if you amend. So he doesn't have to wait to file.
You can efile your amended return. You should not amend until the first return has fully processed and you get the first refund or your tax due payment has cleared.
How to Amend this year
https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-curr...
