You have engaged his teachers, asking them to send you information about his homework assignments and test scores. You need to determine if your son lacks fundamental skills and knowledge in his academic subjects. You must also discover what underlying emotional difficulties might be contributing to his poor performance in school. I would recommend that he see a therapist who is well versed in dealing with boys this age. He needs to be able to trust someone who will not shame him for failing to live up to your expectations (and perhaps his expectations, as well).
I don't believe that you're "losing him," but I know that you must change your approach to helping him with these problems. Right now, all he probably feels from you is your disappointment in him. He knows that you think nothing less than A's and B's are acceptable, so he may think that you believe that he is unacceptable to you.
His teachers should be able to provide you with explanations of why he is doing so poorly academically. If they can't, please have skilled tutors evaluate his knowledge in his academic areas. You owe it to your son to help him without blaming him. This good boy can't do it by himself and right now he's scared that things will always be this bad.