I have to jump in here because your son reminds me a lot of a girl I know very intimately. She started out in elementary school kicking butt and taking names, even if she stared dreamily out the window once she "got" what was being taught.
She pulled down straight A's pretty much through middle school. Scored in the 99th percentile on her tests, took advanced math and other such classes and enjoyed being challenged.
Then in high school, despite being in honors classes she got bored. The challenge wasn't there and she didn't see the point in doing assignments she felt she could do with her eyes closed. So her grades slipped, her chances of getting into a good school and scoring scholarships disappeared.
Fortunately she was super into computers and taught herself web design, and after graduation worked in the SF Bay area until the dot com collapse. It was until she moved home and went to the local community college that a counselor suggested she might have ADD. Apparently it goes unnoticed a lot if girls because they don't tend to have the H part of it, hyperactivity.
So while I know everyone wants to slap the ADD label on kids it might be worth looking into. There are non-stimulant meds now, and it can be managed without meds too. Unfortunately for adults with ADD non med management is a catch 22 because it requires really having your sh*t together which most of don't.
I'd take a peek at a book called "Driven to Distraction." It made me cry and cry and cry my eyes out because I realized that perhaps my life could have been a whole lot different.
Anyway, I wouldn't rule it out. There are a lot of stereotypes about what ADD is and isn't, but that book will give you a good idea. Some people think it's about focus, but really ADD people can focus like mad, but typically on the things they want to.
"True, but if you throw your panties into the middle of the peloton, someone's likely to get hurt."