Just my two cents:
As always, any new console is only surrounded by the hype that limited production creates. The PS3 didn't meet pre-order demands when it launched, and the ridiculous hype made around its scarcity for the first three months were having people paying outrageous prices for them on eBay. So, the XBox One doesn't seem to be generating the same hype because pre-orders didn't sell out right away (mostly due to the PS4 being launched at the same time and the because of all the dumb choices Microsoft promoted that their console would do).
I don't think the One will be backwards compatible with 360 games, so your son's older collection will just sit there. The launch titles (new games) for the console is pretty limited for the time being, so it's not like there's gonna be a whole lot of games released for the One that he's gonna be able to play. And a lot of new games coming out are still being released for the 360/PS3. So even if he wanted Call of Duty Ghost, it's coming out for all systems and he won't miss out on it by not having a One.
I've been a gamer for 20 plus years, I've worked for videogame stores and directly for a division of Sony's Playstation department even, and I know the "hype machine" that surrounds consoles pretty well. Having the new XBox One would be a nice social gift for him, being able to say he has the console. But its actual usage will be pretty limited at launch. I think there are only 2 or 3 exclusive games being released, with another 3 or 4 games that will be released on other consoles as well - and I think they're all basic first person shooters without a whole lot of variety to choose from. Plus, the atrocities that Microsoft are implementing with the One pretty much tells me they're idiots and are looking to milk people out of their money. If you do just a general Google search of "XBox One Problems," you'll see plenty of write ups that'll prove how off kilter they are with their business model.
I'd say this should be a birthday gift for your boy IF his birthday is still months after Christmas. Let all of the reviews come out for the console, see how Microsoft deals with fixing all the bugs and tweaks that it will need, and let them get a good library of games for it before you commit, imo.
Here's a good article to start you off with about the immediate problems with the XBox One: