The deadline this year is July 15.
Q. Can I send (by paper mail) his amended return and my tax return at the same time?
A. Yes. But don't mail them together. You do not have to wait for his amended return to be processed. He needs to amend to totally remove the child.
It appears you understand this, but I'll repeat it for others reading this: If you and the other parent live together, either one of you (but not both) may claim the child. You may decide between you which one will claim the child. Only if you can’t agree, do the IRS tie breaker rules apply, to see who has first choice. It may be worthwhile to prepare trial returns, both ways, to see which way the family comes out best. This tool may be useful: https://turbotax.intuit.com/tax-tools/calculators/taxcaster/?s=1.
