Sorry to hear that Rico is feeling so poorly!
I don't have any experience with stomatitis but another member here found that Atopica helped her cat with stomatitis. I think for some cats, extraction is recommended. Did the vet suggest anything for pain? Maybe it's pain and not nausea that's preventing him from eating?
How much Ondansetron are you giving him? 2mg? You can give it a often as every 8 hours, if required.
200 ml of fluids is way too much. For a cat his size, I wouldn't give more than 100 ml/day. You want to give that as 50 ml twice a day - how is his heart?
As for crystals, the fluids will help with that too.
Are you seeing and difference after his getting IV fluids yesterday? Dehydration can also make them feel terrible and affect their appetite.
Do you get Hills a/d food or other "recovery" food there? They are higher in calories and the texture is easy to syringe feed.
200ml is way too much!