Box 10 of the W-2? Wow.
I would show a copy of the instructions for box 10 of the W-2 to your employer, and ask your employer again to correct it.
https://www.irs.gov/pub/irs-pdf/iw2w3.pdf#page=17
Is your employer just a single individual? Or is it a company that you may be able to speak with somebody else about it?
Otherwise, you can follow the instructions here:
https://www.irs.gov/help-resources/tools-faqs/faqs-for-individuals/frequently-asked-tax-questions-an...
