My poor DD had gastro not long ago and got the sore bum from it. I used Sudocream on her for the rash - it is easier to spread but stays on longer than things like curash. Zinc is the best for staying on but is so thick it can hurt to put on
Don't worry about the food yet - when he is hungry he will eat. It took DD a week to get her appetite back. When they're sick and not eating, their tummy shrinks so it can take a little while for the appetite to build back up.
If you're worried about hydration, gastrolite is good but there is a pead version called peadalite (available only at the Chemist). Does the same thing but is designed for kids. We used this but DD prefered the water .

Hope your little one is better soon.

MG