What age are the kittens? Does the kitten show any signs of diarrhea?
I would also recommend putting Karo syrup on the gums for a jumpstart, and also you could try placing him on a teat while the others are sleeping, so that he is still getting the nutrients from Momma's milk. The other times supplement him, and weigh him daily, he should be gaining about 10 grams a day.
Is the kitten dehydrated? You can check by pulling up the skin on the scruff of the neck and see if it goes back or stays pulled up a lilttle. If he is dehydrated, you will need to either give him some pedialyte diluted with 50% water or if he is very weak, he will need sub-q fluids from your vet.
Around 3-4 weeks old is a good time to begin deworming. The store bought brands, I don't believe are very effective or safe. I recommend Strongid, you can take in a stool sample to your vet.
Good luck, we hope he begins feeling better soon.