I didn't get a chance to read through all the replies but your son sounds a whole lot like mine. He was certainly aware that he wasn't meeting expectations like he was supposed to and it made him feel bad because he just couldn't. This was in a Montessori that I thought was going to be the answer for him (we knew he was super bright and was starting to have issues even in Pre-K). Alas, it was not and this coming year he will be going to a public gifted elementary. I'm hopeful the teachers will be more understanding there and will better be able to engage him in the lessons. He is going into 2nd but last year as a first grader, they had him doing 3rd grade+ work.
He also has visual processing issues so that combined with being EG/PG led to a subpar "performance" in school. Being bored or having vision issues or learning disabilities can certainly lead to behavior that others see as ADHD.
There is a book about misdiagnosing gifted kids with ADHD and it goes through other possibilities. Pretty interesting.
http://www.amazon.com/Misdiagnosis-Diagnoses-Gifted-Children-Adults/dp/0910707677
