@Dannyapp generally, you primarily pay income tax to the state WHERE YOU PHYSICALLY WORK.
If you live in another state, you also have to report your income and pay tax to that state, but they will provide a credit for the tax paid to the state in which you worked, so that the same dollar is not taxed in two states.
So since your son is physically working in IL, then he pays income tax to that state. It is that simple.
