Very good advice above.

I have found that combining Sigmacort with canesten, that'll treat just about any nappy rash that doesn't involve a bacterial infection (and if it doesn't start getting better after 24 hours with these I'd be going to the dr to check). Both available over the counter, I apply one after the other and with a barrier cream.

I also rinse off DD's bum with lukewarm water (not in the bath, just on the change table) when she's rashy, seems kinder than wipes.

Other things to think about:
Have you fed any new foods/a lot of a particular food lately? Bad nappy rash can be a sign of food intolerance. It could even be something seasonal like summer fruit, kwim?
Have you changed nappies/washing powder/wipes/bath lotion lately? Some brands set some kids off.

Good luck, it's such a horrible thing, poor little tike must be so miserable!