Get him into a training ring with a nice soft, deep, sandy floor and just buck him out. Once he figures it's a lot of work to buck and you're still up there, he'll stop...that is, if you can stay in the saddle. Just put a halter on him, so you don't hurt his mouth, keep a tight rein, and keep his head up.
When I was a kid (10 years old) I had a pony that wouldn't ride double, so my younger brother and I decided we'd buck her out. We never did break her. I couldn't stay on long enough! I can't tell you how many times I came off the back end of that pony! We had a ball, but she wouldn't ride double until the day she died.
Contrary to popular (modern) belief, there is nothing wrong with bucking a horse out. There are a multitude of ways of training a horse. They all work to one degree or another, and most will work better on one horse than another. Horses were rough broke for a loooooong time before Parelli came along.