I wouldn't get too invested into new stock for your boy unless you are buying the barreled action from Brownells. My son used the factory Houge stock at the age of 8 and is still using it at 14. Even if you had to chop an inch or two off the factory stock, it would be both cheaper and lighter than a chassis system. When you proof the system and make sure it is a shooter, you can chop the stock for him and use an over the butt slip-on Limbsaver butt pad to extend the stock an inch or so for you to shoot. If money isn't an issue you can go right to the Elf Owl but I'm not sure it is worth the squeeze.
Jay