I am sorry to hear that your kitten has some looser than usual stools.
We can see loose stools after a worming, but I would expect that to happen within 24-48 hours of the medication being given. This many days later it seems unlikely that his loose stools are due to the wormer.
Diarrhea in kittens is often due to parasites, eating things that they shouldn't and eating too much table food or treats. Sometimes we do see loose stools in response to food sensitivities, or a sudden change in diet. If you are switching flavors of food that may be the issue.
Ideally you should have his veterinarian check at least a couple fresh stool samples for parasites as parasite eggs/cysts are shed intermittently and could still be part of the trouble. They may be picked up on the second sample or third test if the first is negative. I realize that he's been wormed, but no one wormer covers all parasites. And there are protozoal parasites that can cause loose stools too. Parasites are a very common cause of loose stools in kittens.
If he still feels well in general now though we can try some things at home to help with the diarrhea in the meantime.
The prescription bland diet foods are not very tasty. You can try offering him a bland diet of 2/3 boiled, white skinless chicken, all fats and juices drained off the meat, mixed with 1/3 boiled, plain white rice. Feed several small meals a day. You might wish to add 1/2 teaspoon of canned pumpkin (not pie filling, just plain pumpkin) to each meal if you are seeing mucous in his stool as fiber helps soothe an irritated colon. Once he's eating that well warm a small bit of the prescription food in a microwave and add it to the homemade bland diet to get him eating a more balanced food.
I would add probiotics designed for cats to his bland diet like Fortiflora to rebalance her gut bacteria, as the bacteria population is often off with diarrhea. Other options are Proviable or a probiotic paste called Benebac, which can be used in dogs and cats.
If he has a tense painful abdomen, continues to have loose stools with no improvement, becomes very lethargic, begins vomiting or runs a fever greater than 103.5F then he really must be seen by a veterinarian as soon as possible. Make sure to take a fresh stool sample with you when you go.
Once he feels better (no diarrhea for 48 hours) start mixing in what will be his regular kitten food very slowly, less bland more regular with each day. It should take a week or so to convert him.
Most cats can, and should, eat a primarily canned food diet. It may be that the canned food was introduced suddenly or that there is an ingredient that doesn't agree with him. So I would not stop feeding canned food, we may need to try a few to figure out what works for him. Or his loose stools may not be related to the food, but rather other parasites, which is often the case with kittens.
Please let me know if you have any further questions.