What is this irritation around my cat’s butt/tail?

I’ve been taking care of this outdoor stray cat for about 4 years. He has fleas, and I’ve noticed some irritation and redness around his butt and underside of his tail. His fur is NOT matted.
I’m currently struggling financially and don’t have money for a vet visit right now, so I’m looking for advice on what this could be and what I can safely do at home while I try to find low cost or free resources for him.
I’m working on getting him neutered and treating his fleas. I also want to give him a bath, but because he’s an outdoor cat, I have to bathe him outside. I want to make sure I use the right soap/shampoo and don’t make his irritated skin worse.

What kind of soap or shampoo is safe to use on a cat with irritated skin? Is regular cat shampoo okay, or is there a specific type I should look for?

Could fleas cause this type of irritation? Is there anything else I can safely do at home to help the area?