I wonder how you decided he wasn't mature enough for school? No 5 year olds are mature, and all have their own quirks, habits, and opinions. Kindergarten is a group experience for learning social skills as well as a place to get an academic preparation, so I'd suggest not worrying about "getting him ready", except be sure he knows basics like the names of colors, and if he's interested, knowing the alphabet and counting and recognizing the letters and numbers. But don't worry about it. The kindergarten teacher will be prepared to take a diverse bunch of children and prepare them academically and encourage them to develop appropriate skills, too.
Your child is polite, smart, and outgoing. What a joy! And he'll be appreciated by his kindergarten teacher for these qualities, too.
Talk school up to him. Kindergarten's not all about academics (and if you find it's that way in your school system, they're not doing it right -- there should be a balance of body movement, academics, arts, free play, etc.). Your child will take to kindergarten best if you talk positively to him about it, or even take him to visit a kindergarten classroom and watch.