How to pick the right bat

Question
My son will be 11 1/2 this spring during the season, and he is about 100 pounds and approx. 4 feet 11 inches tall.  We just purchased for him a 31 inch, 20oz bat on sale which was a great deal, however, I am wondering if the length is correct for him, and also, is there such a thing as having a bat that is too light?  Is there any truth to being able to hit the ball harder with a heavier bat?  Thanks.

Answer
I could not tell you if the bat is the correct length for him because it really does not matter about what size the kid it its how strong he is in the wrist.I do agree that I would rather a kid swing a little heavier bat while choking up on than swing one that is too light.