Go with WiiU ;)
I think PS4/Xbox One are more for mature gamers...
Assuming that they will keep these consoles for 5 or so years, the 13-year old will be an 18-year old at the end of that span. I think he will be mature enough for one of the "big" consoles. Unless he is a big Mario fan, he will be disappointed with the WiiU when he cannot play online with his friends.
The other option is to get both, but then that limits you on how many games you can get.
Well they arent really struggling for money...In a couple years if he wants a ps4/one, then buy him one but for now, I really don't see many games suitable for kids on either of those consoles...
@everyone: Is it just me who feels buying M rated games for someone else's kid is generally a bad idea?