I'd check out his stomach and legs and see if there are any broken veins visible there. If so it might be red leg. But hopefully it's just a minor injury. It is okay to apply a little (nonpainkilling) antibiotic ointment to a frog's injury.

A vet visit might not be a bad idea if you suspect red leg, especially since it's serious and tends to come on quickly. Some kind of antifungal treatment is usually needed for red leg.