Feeding him sour cream is not giving him any vitamins or proteins he needs to be healthy. Take it from some one that knows cats, get some bitches milk from a pet store and feed him that. In a week get kitten chow and mix a little with the bitches milk and let it sit till the chow softens up to a mush and let him eat that. Whole milk and things like sour cream are not really good for them at that age. Well not at any age but especially when they're that young.
And kittens are not ready to be taken from their mom until 8 to 10 weeks old. He's not getting the natural antibodies from his mother's milk. Make sure you get his shots when the time comes. No flea sprays or powders and for sure no collar. The poisons they use are too strong for a little kittens that young. Just keep him brushed out.
He is cute. And they do make great companions. But like I said you got him way too young so he's going to be high maintenance for a few weeks.