Check the tree size of the saddle, and ask your instructor or any knowledgable horseperson at your barn what tree size the horse SHOULD be using. If it's the wrong tree size for the horse... problem solved. He needs a different saddle.

You might also request to not ride in the dressage saddle? Is there another saddle you may be able to use? If so, I'd try the horse in a saddle that you're sure fits him. If the second saddle you try fits him and he STILL bucks, I'd take a look at his back. It may be sore.