does the horse have cold back syndrome?
when bucking, turn him in tight circles. this will preven the horse from bucking.
Don't be scared ride on but never fall off
Make him go forward horses that are truelly going forward can buck
You can't really fix it. If it's something he doesn't normally do then he could just simply be having a bad day, he could be in a bad mood.
If he continues to do it then check to make sure he hasn't become sore anywhere, make sure his hooves are all good. It could also be something that you are doing that he doesn't like, so when he kicks/bucks try to think of what you just did and maybe change that a bit