As someone who's currently chasing the dream and can remember the enjoyable age of 10, I would definitely nurture the ambition, but also remember he is just 10! With another 8 years at school, if he's anything like the people I grew up with he'll change a lot over that time!
I really started researching properly into my options at 16/17 years old- the aviation industry is such a volatile one that in 8 years (assuming he doesn't go to university) it could have completely changed. Currently, military flying is struggling (from what I hear from those in it)- funding isn't at its best and waiting times can be frustrating.
Civil flying at the moment doesn't seem to be that much better- but again 8 years time, who knows! Just looking around this forum you'll find lots of useful information, have a look at modular vs integrated, it will give you an idea of the two ways of training to be a professional pilot.
IMHO, you've got lots and lots of time to think about what he wants to do, regardless best of luck to him
