I'd take him to the store and swing a few like FTW said. It really comes down to a feel thing for your son.
Some leagues due limit the minus number or weights. It's pretty tough to get a bat that will last for more than a couple years due to the growth of the kids.
As far as make, pick one that you can afford. Remember your kid is going to out grow it and it's only little league ball. Hell he may even take it out and ping a few rocks with it when your not around. I've seen 8-9 yr.olds with a bat bag filled with 2 or 3 bats that may cost $75-$100 each. That is just plain stupid. I think the parents buy them to impress the other parents, especially when your talking about traveling tournament teams. If they can afford a more expensive bat fine, but a kid doesn't need more than one. If a kid is a good ball player the bat isn't going to make a difference and vice versa if he's not.