I personally try to go opposite for some brands. For Union, I sized down because the L/XL barely would cinch down even when the ladders were fully shortened. However, my Targas last year, I went L/XL and they fit a 9.5 Nike ZF1 and DC Park fine without the ladders still half slack.
If he is wearing a 10 mens shoe, you'll have to fit him in a boot before you order. He might be a 9 boot, he might be a 10.5 boot from brand to brand. If he's a 9.5 or bigger, go with larger size. If you adjust the ladders on the insides, they should cinch down all the way with extra to ratchet if they pack out. Then when he boots up again next year (the joys of growing), he can keep them. The other thing I noticed about smaller bindings (I sized a pair of Raidens down) is while I could adjust the ladders to get my boot to fit and have the pads/caps centered the bindings were narrower than I liked.
However, it will be no fun if he has to ratche them down all the way and they start to pack out and he's riding loose. Keep in mind, this is possible unless you match the binding in the boot in person (or know of someone who has the same combination of make/model/model year).