It may be his earnings are lower than the federal tax withholding tables dictate withholding be done. You may want to ask them if that is the case here. There is a consideration of how he filled out his W-4. For example: Married filing jointly vs. married filing separately. Here are the tables provided for employers, if you would like check his earnings amount:
https://www.irs.gov/pub/irs-pdf/n1036.pdf
If that is not the case the Department of Labor may be able to help you. Here is a link to their website:
https://www.dol.gov/agencies/whd/contact/complaints#:~:text=If%20you%20have%20questions%20or,trained...
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