Rash near street cats under tail

I was petting this one friendly neighborhood cat that my mom feeds everyday. And when he went to walk towards my mom, he had his tail up, and i noticed under his tail, right near his anus, he had a red circular rash.

What could this be? Can i do anything to help him?

If he lets you handle him, you may be able to help him. You'd have to get the rash diagnosed properly though so you'd know what he needs.