Let him try it before you cut the stock. The 30-30 kicks less than the 35, and with the managed recoil loads it kicks even less. This way when he grows you'll be able to add a recoil pad to lengthen the pull. The 336Y is a great gun, I know of guys who bought it for the shorter barrel then replace the stock with a full sized one. Check out the marlinowners website for lots of great info about your guns.