Since your employer will not issue a corrected W-2, you can manually calculate and update the W-2s so that the correct amount of Social Security tax is being reported. Then, print and mail Form 843 to request a refund of the excess.
Take a look at the following TurboTax help article for detailed instructions and links to Form 843 and its instructions. See the section under What if my employer can't or won't cooperate? under the Single Employer option.
Can I get a refund for excess Social Security tax withheld?
