How can I teach a walker pony not to over jump?

[Horse Help 911!]

I'm riding this TN walker pony that is 14.1hh and very narrow but he jumps like he has springs in his back legs. He clears 5' free jumping and he is only 5! But anyways he over jumps everything. We are showjumping at 2'6" and 3' on Sunday but it is so hard when we sometimes get a bad approach and he squats and leaps, cuz he jumps the 3' like its 5', literally we've taken pictures while he's in the air. Problem is he throws me forward like everytime and there have been many times when I have gone over his neck, part of the problem of my balance is how far forward my saddle puts me so we're gonna put a different saddle on him today. But anyhow, how can I teach him to clear 3' but not to overjump it so much?