depends a lot on who he knows has which console, but the PS4 is a more powerful console in terms of computing power and has a larger number of users so a better online community, but you do pay less for a Xbox, neither are more future proof than the other really. People will argue for hours on which is better but there's very little between them, for a 9 year old I'd absolutely go with whatever his mates have so in your case Xbox I guess.
I have a PS4 but not biased either way as I did originally order an xbox one but they fucked my order up on the first day. I would agree, its about what his friends have. I actually liked the xbox live system better than Playstations, it was easier to use and more intuitive.
As for future proof - both are not and the introduction of the new ones proves this. Just goes to show that both companies are out to make as much money as possible by tagging on small extras as they know people will buy it. I remember Microsoft launching their 2nd edition 360 stating that it guards against the ring of death - surely this shouldnt be an upgrade but a given!!!