I've never used it myself or looked closely at the ingredients but I've noticed that there are quite a lot of ppl on here who don't rate Hills at all... recommended by vets or not. Also a lot of ppl say that dry food isn't very good for cats as it contains a lot of carbs (mainly cereals) that cats don't need and also they should be getting moisture from their food as they don't drink much naturally. Apparently cats on exclusively dry food are constantly dehydrated.
And I must admit I feed dry food quite often (ours are on Burns Ocean Fish at the minute) and my kits do drink a lot. So while for economic reasons I wouldn't throw away the Hills I think might be good idea as you suggested to leave it out for the moment and maybe reintroduce it bit by bit once he's 100% again; if you're feeding him chicken and fish he's probably getting enough... and of course another reason for cats being sick is overfeeding... cats won't usually over eat but kittens often gulp it down especially if there's other animals in the house (competition). Also as he just had the op on (Thurs?) so I expect he had a fast before it, so I bet he gulped his food down after! Try measuring his food (I blagged a free techni-cal measuring cup from the pet store which guestimates weight of dry food) and feeding at 4 regular mealtimes rather than leaving it down. He's probably just been a bit greedy.
Some other signs to watch out for I think would be if his coat seems unhealthy (parted, lank) if you see a lot of his third eye, if his breathing seems too fast or irregular. Don't be embarrassed to ring the vet and ask "silly" questions, we pay them enough as it is and you're their valued client. But if as you say he seems fine (and I'm sure you know your kitten) then I reckon he probably is xx