I know some people disagree on this, but I would never buy a pair of skates that has already been broken in. You're putting your feet under enough strain in the best of circumstances. Someone else's skate is going to be broken into the shape of their foot, which means you're going to be asking for blisters and it will never fit quite right.
At an absolute minimum, go ahead and buy sole replacements at the same time you buy the skates. Your feet will thank you later.
Honestly your best bet is just to go to a used sports store that has lots of hockey gear, they will size him appropriately-
This is your best bet, seriously. IMO you at least need to have someone take a look at your/his feet and tell you what size is correct for skates (it's not the same as shoes) and also whether your feet are particularly wide or narrow relative to a standard sized boot. Again, this is one of those things where you never realize how important it is until you mess it up.
Even if you don't buy the skates there, a decent store will understand where you're coming from and give you some good pointers about how to find something that will fit you properly. Literally write down what they tell you so that you don't have to try and remember it later. The little things make a big difference when it comes to skate selection.