How can I tell what size bat my son should use?

Players come in all types of shapes and sizes. Some are short and stocky, others are tall and thin. When I am sizing up a player for a bat, I will have them take a bat and hold it all the way to the end of the knob with their strong hand and hold the bat out straight in front of them. They must hold the palm down and keep the bat up for 10 seconds. If they can do this then usually the bat is good for them. But you want the heaviest bat that you can do this drill with.