It could be a slight sprain, but it’s more likely he stepped on something. I would check the paw carefully for injuries or something lodged between his toes.
Other than that I would watch him for a few days and try to keep him resting and if he is still painful on Monday I would take him to your local vet.
DO NOT WAIT TO SEE THE VET GO NOW OR CALL NOW PLEASE. I WISH U SO MUCH LUCK ON UR KITTEN!