He spooked, and in that video, he has absolutely no pain. His little 'head bob' at the end has little to do with him being done... horses raise and lower their heads to get something far or close in focus in their eyes. He's trying to locate what bothered him so much. Right before he bucks, he lifts his head up (which COULD be a sign of pain) but his ears are focused away towards something and not back at you (so it's clearly not pain. Horses who raise their head in pain have their ears back, or pointed to the source.) Then he snorts (more fear) and raises his tail to attempt to poop (horses empty their bowels when they get scared, so they're lighter... No joke. Scientific fact. Ah, the wonders of my Equine Science class.
Also, what you mentioned about posting.. If you're a green or a just-getting-back-into-it rider, you could be causing him pain by posting if you're landing too hard in the saddle (hence why he prefers two point). You don't 'sit' in the saddle when you're posting, you softly touch back down. Pretend there are egg shells on the saddle, and you don't want to break them.
He could be in pain, but this video does not show any signs of it. Good luck!