Return using HOH/FTC rejected by IRS

Dual US/CAN citizen, never worked in US. I submitted my initial 2023 return using FEIE as I have done in all previous years, but my son was born Dec 2023 and I discovered I could use FTC to get the child tax credit. Got my son's SSN in July 2024 then submitted an amended return for 2023 using the Foreign Tax Credit/Head of Household. Almost two years later now I get a notice saying I owe the IRS the amount of tax for 2023 that should have been zeroed out by the FTC plus interest. The IRS agent I spoke to said that because my son's SSN was issued after the tax filing date, I cannot use HOH, thus invalidating the FTC? I use HRBlock Expat to submit my returns and reached out to them but haven't heard anything yet. Any suggestions how to tackle this? Can I go back to FEIE for another 2023 amended return?