Frustrated and I hate it!

As a mother of 3 kids I have been through my ups and downs. Even with two kids who were born with medical issues I feel like I have had it pretty easy. My kids are smart and well behaved most of the time. Of course, I am biased as they are raised by a smart and well behaved mother. Lately though, my oldest child is making me more and more frustrated. Just a back story about him. He is going to be 12 years old and entering the sixth grade. He is considered exceptionally smart and knows more about things that most people wouldn't even think of. For fun he will make up math games to play or set up a full football season and then run the games in his head. I am also pretty sure he has read more books than my husband and I put together and we both enjoy reading. The good thing is besides being smart he is athletic and is involved in multiple sports throughout the school year. He is a wonderfully social kid and extremely nice to everyone he knows so why am I so frustrated? Ever since Drew was a toddler I have been told that he needs to be tested for the dreaded "ADHD". When he was young I refused it. I always knew he was ahead of the curve considering we were able to hold full conversations with him since he was 1. Now don't get me wrong I know every parent believes that their kid is exceptional and I feel that every child is in one way or another so I am not down-playing any other parent out there. We just knew early on he was a different kid. Once he hit elementary school I had a lot of parent meetings. Never as bad as they could have been. Lots of positive feedback on how smart Drew is and he is a wonderful student. There was always the dreaded "but" mixed into all the praise. He always did his work very quickly and correctly, but he was disruptive to his classmates when he was left with idle time. He always had something to say about everything and many times they were not at the right time. I finally broke down when he was in 3rd grade and had him tested and the doctor said he was perfectly fine. He had no deficiencies in school and there was no indication that anything was holding him back. So, as a parent I tried to come up with alternatives to help him learn to control his impulses and constant talking. We also got really lucky with some great teachers who understood how to reign him in if he started being distracting to others. So, why am I so frustrated now? School starts in 4 days and I am worried about him. I have found myself getting angry at his constant comments and talking all the time. Many of it is at inappropriate times or things that have nothing to do with what is actually going on at the moment. I do have to remind myself that he is never trying to be disruptive and I do feel like he has so much going on in his head that he feels like he has to say what is on his mind. The problem that I have is a fear that he is going to suffer from his lack of verbal self control. He also has a tendency to be shunned by peers because he talks out of turn or over people. Plus, I feel that he isn't always going to get a teacher who understands him and that he may lose his love of learning because he is in trouble more than he is being praised. I am really hoping that all my fears are foolish and he proves me wrong by having a wonderful year in school.