My son still has his driver's license and license plates from IL even though he is now working in Wisconsin. Is he still considered a resident of Wisconsin?

If he has not transferred his drivers' license and plates (and, possibly, voter registration) to Wisconsin, he is still considered a resident of Illinois and would file as a non-resident in Wisconsin.  He may have to file IL taxes and show what he paid in Wisconsin.  This varies by state.

generally, driver's license, voter's reg  etc are used to determine the residency of the taxpayer -- closer connection.

Additionally IL and WI being reciprocal state, the state of residency taxes income ( generally wages/self-employment etc. ) no matter where earned.