@Hal_Al is correct. However, as a second year medical student, if your son's permanent residence is Iowa, and he lived there all 12 months of 2020, then he would only need to file in Iowa and not Ohio.
If he voted in Ohio, has a driver's license from Ohio, and uses your address for official mailings, then he would file in Ohio and get credit to the extent of the Iowa income taxes paid.
Specifically, he would be adding Ohio first in TurboTax and completing the Ohio interview before adding Iowa. The sequence is important to properly allocate the income. The residency on December 31 should be added last.
Keep in mind, Ohio may consider him a resident and Iowa may consider him a nonresident. TurboTax will capture the details through the interview. Residency is an important distinction when completing the state returns.
For more information, see:
Income - Ohio Residency and Residency Credits-see Question/Answer #5 and #6.
Are you a resident of Iowa for Tax Purposes?
How do I file a nonresident state return?
