Q. As my son's income was $1,800 (he worked as a teaching assistant in his university), he doesn't seem to need to file a tax return at all, correct? His income is below a threshold.
A. Yes. The threshold for earned income is $12,550.
Q. Does it matter that his job is located in a different state (so, the income was earned in NY and he lives in IL)?
A. Yes, theoretically. But in your case, no. If you work in a different state, you pay tax to that state and your home state. But, your home state gives you a credit for any tax paid to the non resident state. But, the NY filing threshold, for his case (dependent) is $3100, so he doesn't need to file.
Reference: https://www.tax.ny.gov/pit/file/nonresidents.htm
