Hard to say. Kids that age can do so many funky things. Sometimes they even do funky things that work for them. For a while.
It may be that he's trying to pull everything. Try soft-tossing to simulate letting outside pitches get deeper in the zone and have him work on driving those oppo or up the middle.
Somebody may have taught him to uppercut the ball and he's going to far with it. Or he could be hitting too far out front. Or both.
Another thing to watch out for is his stride. Kids that age often fail to get their foot down in time; if that looks late try just eliminating it for now.
Around that age one of my kids had an issue with straightening up some as he swung. This caused the level that his eyes were on to raise in the middle of his swing, which resulted in hitting the top of the ball. You might look for that as well.
You should also check his grip. If it's not close to "knocking knuckles" it could be causing him to roll over. To prevent rollover, one technique that seems to work well for kids that age is working on getting the top hand facing up and bottom hand facing down at the point of impact.