How do you get him to stop...when he is is already going?

I would do a lot of ground work with him on a lunge line and under saddle with full gear on I mean bit in mouth reins tied up so they don't slip.It sounds like some thing he was allowed to get away with that became a bad habit or they just kept him more doing harness work which makes him feel more contained so that's why no bucks or rears at that time.So back to basics and that's ground work.At a lope of course.