whatever fits his foot. either way it doesnt matter, theyre all made of mafrin, which is the cheapest plastic you can make a book out of, it doesnt grind well, or hold a stretch well. plus the liner is gonna suck more than a bit
boots are the one thing you don't want to put a price limitation on, if you need to save money get a lower end binding or an inexpensive ski, or buy used. plenty of good deals in the buy/sell/trade forum.
I don't know of any boots that you'll buy "new" for that price best bet is BST or Ebay/craigslist
have him try on a boot at a shop, then find an older model of find it used.
it is the most important peice of equipment ever, it is your connesction tothe ski and snow if it dosent fit right u will ahve a terrible season cuzi t will be uncomfortable and you wont wanna ride, if their too big u wount be able to control the ski and thats dangerous