T.U.
It seems like everyone is saying the same thing, but I'll say it too. Just make it fun. I know he has years of school ahead of him, but why not teach him when he's young? It can only help. Have you tried fingerpainting, drawing with chalk on the sidewalk/driveway, or pudding painting. Pudding painting is just making some chocolate pudding and letting him color with it. If he's like most boys he'll enjoy it as long as it's active learning. Good luck!