When I read your post it was like reading about myself. I was an extremely bright kid who scored high on standardized tests but they wouldn't move me up because I got bad grades.
I can tell you why. I was BORED BORED BORED!
It's hard to sit in a class that moves soooo slloooowwwly when your brain moves twice as fast. I was always in trouble for playing, not paying attention and not wanting to do the work. I used to read the entire textbooks the day we received them and then be bored to tears during class.
Usually what would happen is the teacher would explain something and I'd turn my ears off because I already understood it. THEN, while I wasn't paying attention, she'd tell us something I needed to know that I didn't already know. BUT, since I wasn't paying attention, I'd miss that important detail.
Also, I didn't have a full understanding of WHY I needed to know stuff at school. To me, school was a boring place that moved way too slow and I didn't see a point in even trying.
You'll be happy to know I did make it to college on scholarship (based on my ACT scores) and I graduated from college with honors. College was great, I could move and learn at my own pace.
If I were you, I'd first have a talk with your son. What does he want out of life? What does he want to do as a career? What subjects interest him? Where might he want to go to college?
Then, help him map out a plan to get there that excites HIM. You may need to look to alternative forms of education. I would have thrived in a study at home kind of setup. When I was growing up there wasn't that option. Now they have many study at home options, he can study at his own pace and earn a high school diploma (not a GED).
Check out http://www.k12.com/
They have public and private options. The public school option is a study at home option that doesn't cost you anything (because it's a public school). Or you may want to look into a private option.
He doesn't need therapy. He needs positive motivation and a challenging curriculum. If I had had that as a child I would have flourished. Instead it took me until college to finally get where I enjoyed learning. (Even now I do best in an atmosphere where I learn and move forward at my own pace.)
Good luck, and I'd actually direct you to my mom if you have any questions. She'll tell you herself I was very challenging to raise and you two can commiserate. LOL!
(The good news is your son is smart and he WILL make it in this world! He just has to find what works for him. Gifted children can't be forced to learn in a box. We wilt like flowers with no sunlight.)