Apple

But why does he need a smartphone? Is it because his education demands it or all his friends have one and he wants it?

It is good to keep one updated in terms of technology and content but having a smartphone at such a young age is probably not a good idea. But if he insists, 13 or 14 series would do. Personally, he can make use of the iPad or a Galaxy Tab for both studying and movies or games. I remember sharing my phone with my brother when I was 16. It was a Sony K310i, oh man, the fun we had using that phone, having an IR blaster to transfer mp3 files haha.