The people above me are correct. Specifically, your son lives and works in states with reciprocity. If your son filed the form linked below, then he only has to file in WI.
If your work state has reciprocity with your home state, your work state shouldn’t withhold income taxes. See which states have tax reciprocity.
www.taxslayer.com
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Illinois
Illinois has a reciprocity agreement with
Iowa,
Kentucky,
Michigan, and
Wisconsin. If you are a resident of IA, KY, MI, or WI, but you work in Illinois, income should not be withheld from your paycheck for Illinois state taxes.
Fill out and submit
Form IL-W-5-NR to your employer."