Another non-expert who agrees with the long swing diagnosis. Very common for kids his age. Watch his shoulders and hips at point of contact. Both are at the end of their ranges, so they're adding zero power. Improvement will probably come almost immediately with the right instruction. My advise is to find someone who was very successful with hitting at the college or pro level. Doesn't have to be a professional instructor, just someone who knows hitting. IMO, hitting is much less mysterious than pitching (fewer different opinions about what is correct technique).
In the mean time, google "long swing" and "casting" to get a better understanding. Watch slo-mo of Mike Trout.
If you're anxious to get him started, front toss to him over the inside third of the plate and challenge him to drive the ball to the RF side. That might give him a taste of getting his hands inside and shortening up his swing. Good luck.