In the meantime try as best you can to stop him becoming dehydrated. Make sure he always has water to hand. The downside is that he could be losing minerals from his body and too much water will dilute what is left of them and this isdangerous if it goes on for too long, particularly in such a tiny body. Keep a close eye on him and if he becomes easily tired, stops seeming well in himself and doing usual kitten stuff, take him to the vet sooner rather than later.
Also look at his gums (if he will let you) just to check that he is not losing blood in his stools. His gums should be deep pink, not pale. Kittens can lose blood very quickly as they have so little in their body.
It could be a reaction to the jab, though this is unusual. Also sometimes the wound can become infected after neutering. Again this is rare but if that is the case the infection could be affecting his stomach. This seems unlikely if he appears to be otherwise well. An infection would be likely to make him run down. If it is the case though and he needs antibiotics the vet will more than likely give him an antibiotic injection.
I hope he makes a full recovery soon.