kindergarten evens out quickly in school and play is a type of learning anyway-
i would recommend that you teach him his letters and numbers- parent involvment (thruout school) has a lot to do with student success, so starting now sets good habits. there are lots of educational books, videos, etc. but since you have a younger child who probably keeps you busy, working it into your daily activities is a great and fun way to teach. when you read books show him a few letters each time, get foam letters for the bathtub, make a game of finding the C on the cereal box... or the S on the stop sign. practice counting at lunch- how many pieces of apple/sandwich/cookie do you have? how many stairs at the playground? making it a game and keeping it positive make it fun. Also, the children's museum, aquarium, library, etc. are great places to teach things and he might be having so much fun he won't even realize he's learning.