Our son lives in Indiana full time but his drivers license is from Illinois. Which state do I file a return with?
You file a tax return for the state in which you LIVE an/or in which you WORK. Your driver's license does not determine where you file a tax return.
If you live in Indiana you should, however, be aware of the laws regarding getting a driver's license in that state.
https://www.in.gov/bmv/licenses-permits-ids/learners-permits-and-drivers-licenses-overview/drivers-license/