After you check for pain, and then lunge him at the walk, trot, and canter and then get right back on.
My horse did this the other day. He's 11 and never bucked in his life.. not in pasture, not on the lunge line, and definitely not under saddle.
Just pretend it never happened, so long as there aren't any signs of pain. If there is pain then check saddle fit, teeth, pads, cinch/girth, etc.
Just be careful, if you don't feel comfortable getting back on, then just make him work it out on the ground on the lungeline.
"Strength is the ability to use a muscle without tension"