DS reacted badly to wipes early on. Used a wet thin face cloth for wiping anything messy, but have recently progressed to toilet paper.
What else does he wear on his bottom? Undies? Have you bought new bedding? Have you tried changing washing powders.
DS had a couple of angry blistery reactions on legs and bottom - we now think it was a new duvet cover (he slept with his shirt on, but no pants). We gave it several good hot washes before using it again, and that stopped the rashes (although after that he seemed really sensitive to warm baths for a few weeks - but it is winter here in New Zealand - so cool baths are out of the question). We were just cautious about which soaps we used for a while, and made sure all the soap was well rinsed from his clothes. It cleared up totally in a couple of weeks with no medication required.
The rash was very violent blistery stuff, though. Very scary.