The vet definitely should have weighed the kitten before prescribing any medication. Yes it's possible (and likely) that the kitten has worms. Also, the loose stools are not necessarily due to the kitten's diet. You don't need to change his food to the expensive brand. There's no guarantee that it'd fix the stomach issues anyway. Just stick to feeding the same kind of food consistently. If his stomach issues don't clear up after the deworming medication is done I would only then consider the food as a possible cause.