Why would you buy anything? He's going to be growing until he's 16/17/18. I say, rent each year until his growth stablizes. If you don't rent, you'll be buying a new pair of boot each year for the next 8 years. Just my opinion. Have fun.
EDIT:
I've been riding for nearly a decade, my experience with snowboard manufacturers: Burton and Solomon are the best. Burton boots, bindings and boards are second to none. Solomon makes comparable boots and boards, but I don't like their bindings.
Buy strap system bindings and pleez pleez get him a helmet. I've had two concussions without mine and 0 after I got it.