I am sure that you have communicated your disappointment and disapproval, perhaps always believing that you were attempting to motivate him. What kids take from this kind of ongoing "underachiever commentary" is that they are never enough...not in the eyes of their parents or their teachers. Think about what your son is hearing from you, his piano teacher, and his school teachers: "You could be better than you are." Is this a chorus that should inspire a child and improve his self-worth?
My guess is that you've been expecting great things from your son ever since someone labeled him "gifted and talented" in kindergarten. His aptitude tests continue to show you how great he could be. But he continues to disappoint you and his instructors, not being the A student that you think he should be. It's got to be tough for him to know that he disappoints so many people on a regular basis.
You and your husband have compared your son's lack of achievement (in your terms) with your own past " extremely high potential with low achievement." I think that you see him making the same underachieving mistakes that you did and might be confusing who you were in your childhood with who he is. Your son needs to be viewed independently of you, your regrets, and your unfulfilled potential. Unless you separate yourself from him in these matters, you will not see him for the child he truly is -- a delightful, bright, sensitive boy who is becoming who he was meant to be, on his own terms. I'd recommend reading Giving the Love That Heals by Hendrix and Hunt, to help you deal with the unfinished business of your childhood that is interfering with your appreciating, valuing, and encouraging your son.
I know that you love your son and want him to be happy. Please take the time out to at least consider my comments, as you rethink how you can help him become who he is naturally meant to be. Thanks for listening.