current advise is to carrying on feeding as normal, milk and solids.
DON@T use baby wipes, just water and cotton, or dip him straight in sink.
Let air at bottom as much as poss, put lots of towels or sheets down to protect floor.
I swear by metanium, if using a barrier cream, don't spread too thick, it stops the absorbancy of the nappy so poo sits next to skin.
Sorry no help on cause etc.
Tis horrid when they get sore bums.
My DD had dreadful nappy rash a few months ago, we all had dodgy tummies but it hit her the worst, her bottom was bleeding .
I posted about it at the time and a few people suggested egg white painted on. I tried it and it worked pretty well. You have to let it dry in the air and it creates a barrier.
Isn't advised these days because of salmonella risks or something, but I did it and she was fine!