Are his nails long? Sometimes a rabbit will scratch themselves when grooming if a nail is long, It would be fine to take a little betadine solution and dilute it until it is light pink rather than red. Use the a cotton swab or Q tip to gently cleanse the are so it doesn't get infected. You can also dilute hydrogen peroxide (don't use full strength). Is he alone or with other rabbits?
Looks like a scratch to me