Update: I have decided to keep my horse and try to investigate more as to why he bucked. I have not been back on him but my instructor has ridden him several times at a walk and trot. I am working on more ground work and build up my confidence. I am having trouble now because he stops and doesn't want to lead as he once was. I am going back to the basics and will keep working with him. I am in no hurry and decided I want to help train him so I can learn him and how he thinks.
Thanks everyone for all the great advice. I am sure you will see many more posts from me over the next several months.
Happy to hear you are keeping your horse. I just read the thread and I think this horse could be in pain, kudos to you for investigating that.
I also believe your instructor is not doing any good, it sounds like they are either nervous, or inexperienced and with that said, probably not the best for you and your horse.
You mentioned your horse is usually good at the walk and trot and that the canter is the issue. With your instructor only riding walk and trot, that tells me they don't know how to handle it if a situation arises, and they are letting the horse get the upper hand, because your horse has effectively learned that "if i buck someone off, i don't have to work as hard anymore."
Also, a big tip to see if it is pain related is if the issue is repeatable. If the horse only does it with you, you will know it's lessons and training that you need. If it is repeatable with another rider, or on the ground/ free lunging, it is more likely that its pain, from misalignment, saddle fit, or both.