The previous suggestion is good. Swampy's black cat got stung by a bumblebee once and he also really swelled, but after a day or too it went down and he got over it. The cat learned, that place in the yard with that strange buzzing noise was not a kitty safe area.
You were wise to watch your kitten for swelling of the mouth and face!
Did you also check to see that no "stinger" remained in his paw?
It's been several hours now, so I hope he's better, but if the paw is still very swollen and he appears uncomfortable, he might need an antihistamine or even prednisone to bring it down.
Does you veterinarian have an answering service? If so, it would be good to check in with whomever is on call.
Best wishes to both you and the little guy.