My kitten keeps getting diarrhea! help!?

Have the vet check a stool sample - there are other parasites common in kittens, not just worms. The vet needs to check specifically for coccidia and giardia - both will cause watery poop. You can also try probiotics, which you can get from the vet, or a product called Benebac from the pet store. It's not the food you're feeding, since you're already feeding a corn-free and by-product free food. And he did return to 'normal' after being wormed, so I really suspect a parasite. Take a nice, fresh stool sample to the vet with you on Thursday, and you'll get the correct medication to return him to normal. The probiotics will help though.