I agree about him not being "fully broke." I also suspect the saddle. My horse is terribly sensitive in the back and I have to be careful what saddle I put on him or he will buck like a bronc as well, even walking on the lead.
This guy sounds like a good refresher course, starting with ground work is in order as well as making sure that saddle is fitting correctly.
I would generally be wary of any 3 year old that someone says is "fully broke" because generally it is unhealthy for them to break them before that age. And one person's definition of "fully broke" may be different than another's. I know a person in my area who says she "breaks" horses and she sells her horses as "fully broke." However I used to board at her place and in fact what she calls a fully broke horse is a horse where she can put a saddle on it, sit in the saddle and make it walk around in a round pen...that's it. To me that is merely a horse that is "started."
Training is the answer here, drugging is not and will never solve the problem, just cover it a little.