[JavaBrewer]

Actually the best thing is to get him a t-ball stand and let him practice hitting that first. It's a good lesson that teaches him how to swing straight and directly at the ball. Tennis balls are good for practicing catch/fielding. Don't worry, nearly every kid at this level suck at catching and throwing. If your son can catch 3-4 out of 10 he's doing fine. Practice practice practice.

Whiffle balls are awesome when your son moves up to a slow pitch machine ~ 40mph. You get to sit in a lawn chair with a beer and throw them all day. I also highly recommend a batting cage. My son's team spent a ton of time in the batting cages this wet winter and as a result they are all hitting regularly.

Take lots of pictures!