Awww, aren't you kind to take this kitten in
Which Wellness are you feeding him? It sounds like your kitty needs a very low carb, grainfree diet. The cans of Wellness with the yellow triangle are grain free, but do have sweet potatoes in it and some kitties can't even tolerate that. Kathryn is correct all hard food should be discontinued as it is high in carbs, cats digestive tract is not made for carbs.
Another good food to try is Nature's Variety Instinct canned. It has many flavours and like Wellness, it is about 95% human grade meat, but doesn't contain sweet potatoes.
In the meantime, Slippery Elm Bark is an excellent product that coats the intestinal tract. I find it works wonders on Jasper who can't tolerate carbs. I sprinkle it on water down food and the kitties don't seem to mind that it is there
Here is some good reading: