Where does my son file taxes, and can we claim him?

I live in Illinois. My 21-year-old just graduated from college in Iowa, and stayed there.

He graduated in May, and was able to stay in his off-campus housing (fraternity) through June. He did not have a job at that time, and we paid the majority of his living expenses through June.

At the end of June, he got a job in Iowa. At the beginning of July, he signed a lease and moved into an apartment.

We signed our car over to him, and he has now (in August) registered his car in Iowa, and he has an appointment to get his license next week.

At what point was/will he be considered a resident of Iowa?

Obviously he will need to file taxes in Iowa, but will he also need to file an Illinois tax return, if he wasn’t considered an Iowa resident at the time he got the job?

I read that due to a reciprocity agreement between Iowa and Illinois, that if his employer withheld Iowa taxes during that time, he’d need to file an amendment in order to file in Illinois. This sounds like a pain, but we’ll have him do it if necessary.

And how do we determine if we can claim him on our taxes? As mentioned, we paid the vast majority of his living expenses through June. We still have him on our medical insurance, we pay his cell phone bill, we paid his car insurance (both under our plan and now his own plan), and we paid for his vehicle registration.