I'm not sure what you should do other than keep in contact with the vet about his symptoms and take him back in if you're worrying, but it's important to make sure he's getting enough fluids if he's being sick. (the vet should give you a syringe feeder if he isn't drinking himself.) Out of interest, what food have you got him on? a bit of boiled chicken and rice or decent wet food might be better than biscuits especially while he's needing the fluids
The other thing is (although he does seem to have been sick rather a lot) sometimes cats are quite often sick for apparently no reason. What does it look (and smell) like? The constistency can tell us quite a lot about the health of the cat. If it looks like mostly undigested food and doesn't smell too bad, it's not usually a worry, they often just barf up their tea and eat it later :/ if it's grass and phlegm he may have eaten the grass to make himself vomit as he felt a bit horrid from the drugs. If it smells like poo get him to the vet right away as that could indicate serious problems.
Apparently some cats are just sick for attention which is a particularly revolting habit.
I hope it's nothing serious and your little man feels better soon, I have a brave lil soldier of the same age who had the snip yesterday so *empathy* going your way xx