Hi, My Son Has Had A Brilliant Red Rash On

Hi, my son has had a brilliant red rash on his but cheek for almost a year of and on. It's not diaper rash, and he has eczema, which it what I believed it too be. However, lately it's back with a vengeance. I use a special barrier cream on his bum every night. Lately that's done nothing, I've used cortisone cream, also done nothing. I have used essential oils, that made it worse. We don't yet have insurance so I don't want to drag him to the doc if it's not necessary. It is painful, but he says it doesn't itch. Any ideas what this beast of a rash is, and any ideas of how to calm it?