Thanks for the tip Maz - never thought of that.

It was a disposable nappy pant type nappy, the ones that've been advertised on TV recently. I was testing them out for night use and he had it on for about an hour. Back in cloth since and it's much better than it was yesterday. Has some patches of normal skin but still some bright red areas. Cortisone sems to be doing the trick.

It was so bad though, more like a burn than a rash, bright red and spread all where the nappy had been including his testicles. He's been able to sit down and walk today which is a huge improvement! Poor little guy.