L.G.
I had a similar experience with both of my kids. My son is the oldest so went through it first. He reminds me of what you are going through with your son except mine wasn't a Casonova---lol. I bent over backwards trying to get my son to study more, grounded him for bad grades, took away video privledges, etc. Nothing worked. He never did have to repeat a grade but he came mighty close. He did somehow get through high school (Catholic High), passing by the skin of his teeth. My son was always smart and it embarrassed me that he did so poorly in school. Now he is in college and doing great. I think it's because he is now studying something of his own choosing that he is actually interested in (digital film making).
My daughter, who is 4 years younger, gave me the same kind of trouble starting at about 7th grade. I responded to her differently though, AND had a different outcome. Instead of hounding her, grounding her, etc etc, I pretty much stayed out of it. She failed 9th grade honors English, had to go to summer school, got kicked out of the honors English program, and ALSO was not able to get a driver's permit (you have to have a "C" average now to do this). Suddenly, this year she is studying all the time, turning in all her work (a first for her) and making good grades for once. All this with no interference from me. I think it was the fact that she suffered a painful consequence of her bad grades that turned her around. I never let my son fail---and therefore he never felt pain over his actions (or in-actions). Good luck---I hope this helps.