I know my friends horse does that whenever he's fresh and tries to buck her off. Mine even does it when I haven't ridden him in a week or two and have not lunged him, but not to the point to pull me down to throw me off. Try getting a stronger bit like a correction or low port, set a ground pole after the fence and don't 2-point. Pull his head up after he jumps because if that happens again, you could die or be paralyzed if you land the wrong way. He needs to know that that is NOT okay. Sorry, I don't believe in always being mean to horses, but when they are doing something that could seriously hurt the rider, it needs to be punished and he needs to respect you. Greenies I have seen do that, but not to the point where the rider falls or even gets pulled over. And I have seen a lot of green horses. Possibly even send him to a trainer if he's that bad.