Oooh, poor little one
As long as he is drinking, he will be fine. You might want to use a barrier cream to protect his bottom from so much poo and wiping - he might get a bit red. The only thing you can do for him (besides lots of cuddles!) is keep up his fluids and don't offer any fibrous foods. Once he gets back onto food, just offer white starchy things (banana is a good one, white bread, noodles, rice, dry crackers etc).
If he seems really lethargic and non-responsive, that's when you need to get him some medical help.
The last gastro bug we had here lasted for over a week... but it wasn't as severe as what you're describing. Hopefully this bout will be over in 24 hours - I'll keep my fingers crossed for you.
Only other thing to remember with gastro is to clean, clean, clean. If Natty ever spews I bring out the anti-bac wipes and clean EVERYTHING that she touches. Probably a little bit OC, but gastro germies spread through their poo and spew, so it's important to get rid of all the traces.