The income's source is capital gain (or loss, depending on pending paperwork) and interest earned - both of them in foreign country. (His income for the year could in fact be "negative") The person is Ohio resident and US citizen. What form(s) does he need to file? Can he use the same form to file both federal and state tax? Or does have to file them separately?

Assuming you are a U.S. citizen or a U.S. resident for tax purposes, you will file Form 1040 with Schedule B (to report the interest) and Schedule D (to report the capotal gains/losses). This return MUST be filed regardless of whether you have positive or negative income overall.

The state tax return may NOT be required; it depends on the state and whether you have positive or negative income. If you do have to file, each state has its own state tax forms.