Yes. you missed the checkbox. If he is a dependent check his 1040. By Standard Deduction it should be checked for Someone can claim: You as a Dependent.
You will have to mail in your return this year. You don't have to wait for his amended return to be processed.
He will have to amend his return to say he can be claimed on someone else's return. It will take 8-12 weeks to process an amended return if not longer. In the meantime you will probably each get a letter from the IRS to determine who is able to claim him.
How to Amend the current year
https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-curr...
