My nine year old is about 52" tall and weighs about 70 lbs.. Also this is his first year playing but, he has played football and wrestled the last four years so his doing great so far this season. So, I'm wondering what kind and how to choose the right bat?

Here's an article to read to start with:
Best Way - How Can I Determine the Right Size Baseball Bat for My Child? | eHow.com

I can also guess that your son's coach can give some good advice as to what length and weight the bad should be that would be best for him.