As you may conclude from the other answer, the Ohio filing requirement is wishy washy. The simple answer is he does not need to file a state return if his total income was less than $10,850. Otherwise, he needs to file, even though the income was earned out of state.. Ohio will give him a credit, or partial credit, for any tax paid to Illinois.
If he is your dependent, he must file a city return for the city you live in. Even if he isn't your dependent, he must file a city return for the city of his permanent residence. There is no minimum income filing requirement for most Ohio cities, although some (most) exempt people under 18.