I allow my colts to buck ONE time with the saddle. After that I leave the halter on them and send them around the pen and when they start to buck I pull them around and make them face up then go the other way. It doesn't take long before they just accept the saddle. You don't need to be in a round pen to do it. Put on a rope halter and an extra long lead rope and before he really gets bucking make him change directions. he will soon forget why he was so interested in bucking. If you allow him to buck under saddle (or surcingle) with no consequences then it may get to be a habit and he will need to be lunged before he is ridden every time.
There's nothing like the Rockies in the springtime... Nothing like the freedom in the air... And there ain't nothing better than draggin calves to the fire and there's nothing like the smell of burning hair. -Brenn Hill