If you are claiming your son as a dependent---which you can do if he is a full-time student under the age of 24---and you live in CA, then he is "temporarily absent" from home and is still a resident of CA for tax purposes.
WA has no state income tax. But CA does have a state income tax. He will need to file a federal return and a CA return. You have not mentioned whether he had a job in MI while there for school.
MY DEPENDENT HAD A JOB
If your dependent has a W-2 for his after-school job, summer job, etc. you do not include the information on your own return. You can still claim your child as a dependent on your own return. He/she can file his own return for a refund of some of his withheld wages (he won’t get back anything for Social Security or Medicare), but MUST indicate on it that he can be claimed as a dependent on someone else’s return. (Supervise this closely or prepare it for him!)
If your dependent’s earnings were over $400 and were reported on a 1099Misc or 1099NEC then he must file a return and pay self-employment tax for Social Security and Medicare.
You might also want to use free software from the IRS Free File versions:
https://apps.irs.gov/app/freeFile/
(The MI address used by the employer on the W-2 is not important). When your son prepares his tax return he should use the mailing address where he wants to receive a letter or a check from the IRS.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
