Your child is ill-behaved. It does not matter if he is brilliant or dim. He is ill-behaved and that is where your focus should be. Academic ability is a gift; behavior is character.
More information is needed about your son, you and his father, and the home environment. For instance you do not mention if he has siblings, and, if so, how he gets along with them. Nor do you mention what steps you have taken to train him to be obedient. Children are clever. I suspect that your son relies on his academic accomplishments to escape the discipline he so desperately needs.
Intelligence does not preclude having ADHD. Albert Einstein is just one of many highly intelligent people who were thought to have had ADHD. Intelligence just means that you will do well in school until you hit the point where you can't. That happens for some people about the time Algebra hits. For others, it may not happen until Calculus. You probably should take a look at other symptoms of ADHD. This is a very good site for that.
and this article deals with the gifted student with ADHD.
Obviously the school has tried to discipline him. And you say that you have tried everything. So I kind of question if lack of discipline is the problem. The problem is that if he really does have ADHD, then the normal modes of discipline do not work. I don't know how long ago the ADHD diagnosis was made or the doctor that made it. But given the situation, I would find either a pediatric psychiatrist or psychologist that specializes in ADHD and get another opinion. Then I would get counseling from them on how to work with your son - if he does have ADHD. If he does not have it, then they also should be able to help you.
Finally, there are books like "The ADD/ ADHD Answer book," by Susan Ashley, which give many ways to help the child. And this site also gives some pretty good parenting tips. http://www.rxlist.com/tips_for_parenting_a_child_with_adhd/page3.htm#rewards
Hope this helps. I also post on the ADHD forum if you ever have any other questions. Best wishes.