Mmm, dont’ know really. Could be that perhaps he’s feeling off colour and not driinking enough so his pee is a bit concentrated - hence the rash.
I bang on about this all the time,b ut I swear egg white is the bees knees. Smear some over the red areas and within a few hours there will be a very marked improvement! Coupled with silk liners if you have any for overnight nappy and by the morning it should be virtually gone - egg white won’t help bums where the skin has blistered, burst and is raw & bleeding unfortunately but it will stop it from getting worst.