If he's pooping a lot it's probably that (or at least, it won't be helping!) - the redness is usually caused by the mixture of, er, wee and poo on the skin, isn't it? Even if you get to it quickly, there's still a certain amount of contact time that can irritate the skin. Are you using a barrier cream? They can have reactions to those too, but it might help. Might be worth trying to get some more binding foods into his diet (rice, bananas, carrots I think...) and cutting down on foods with the opposite effect.
At least it doesn't seem to be bothering him, but it's still distressing to see.