Like several have mentioned the nervous/race bred then lazy doesn't really go together.. I'd step back and address ulcers even if they aren't confirmed. (I like Zesterra but there are others) I'd love to see a video of him working.  Then I'd deal with body soreness or pain and then step up the riding and keep doing the drills. If he did them great at the clinic then maybe he is sore..

Agree with all the replies! If he's worth it to you, I'd treat him for ulcers and go get xrays for kissings spine. Rule out the above before calling him spoiled and making him even more sore! (if he already is!) give him one more chance to shine and then I would throw in the flag and let him do something else if he doesn't step it up. My horse has an attitude, but she would never throw me off unless something was WRONG! Personal opinion, if he really loved his job he wouldn't pull that, and if it's JUST an attitude problem, it's not worth the blood, sweat, and tears. 

I would schedule a vet appointment and not try and pin point it to one particular thing. Tell the vet the whole story, let them do a complete and thorough physical examination, and then address the things other people have mentioned.  Laziness is one thing- throwing you off is another. I have had a lot of laid back/lazy horses, but none of them bucked.  It's a lot of work to buck, especially to throw you off like he did. I'd be starting from square one with a vet who does a lot of lameness/sports med work and see what they can find. 

what did Ron Ralls say when you asked him?

I'd definitely look into ulcers.  And I agree 100% with the above about PSSM2.  It is a muscle disease (not like PSSM1, which is a starch/sugar issue) and usually affects them later in life, anywhere from 6 up.  Refusing, "sticky" legs, bucking, spooking, bolting, hard to hold feet for a farrier - are a few of the symptoms.  Some have muscle episodes where the muscles kind of quiver/move - almost like shivering.  Some tie up.  Some just get nasty.  All due to pain.  Join the PSSM Forum on Facebook and pick brains on there.  Post his breeding (a picture of his papers or the link to allbreed pedigree) and some of the knowledable people can see if he has suspects in his breeding.  There are recommended diets too that can help many of them but there is no cure.