I expect him to have the size in the next winter season of 5 feet 10 inches, about 145 pounds, size 9 boot.
My suggestions would be a Salomon Huck Knife 152 if you don't expect him to weigh more than 150 lbs anytime soon, or a 155 if you expect him to weigh more after next season. Similarly you could go for the Salomon Sleepwalker 151 based off of his anticipated size for next year, or a 153 or 155 if you think he will weigh more than 150lbs in 2 or 3 seasons. You could also go for a K2 geometric in 152 or 156 depending on his anticipated future growth. The Nidecker Merc 152 or 156 possibly.
With his current size he could probably ride the very largest size available in boys boards if price/money is a major consideration but he may only get one season before he grows too big, so it is probably best to go ahead and buy a mens board for more future use. Boot size is not going to be a problem/consideration for board width, with the caveat that if you go with a boys (not mens) board that a wide would be preferable only in that case.
He is probably right on the border of the smallest mens boards and biggest boys boards right now.