Good golly, 9 and he is already pulling teenager stunts.
The kid sounds pretty sharp. He has certainly figured out how to get a rise out of you. The smirking may be just that. He has figured out it confuses you - makes you change your mind or how you are going to punish him or its just his way of a nonverbal answer. Or it maybe that he's not real sure how to act, and its a cover up. No big deal either way - IGNORE IT. His smirking is not the problem, it's what he has done before the smirk. That is what you have to deal with it.
What you really have to do is to be very consistent in dealing with him. Also be polite and communicate why. Don't yell, swear, or show disrespect. If he has really got to you - give him a time out and tell him that after you have had a while to think about his consequences, you will deal with him. It will drive him crazy, and give you time to compose yourself and deal with him in the manner I described above.
You probably won't see a change overnight, but if you consistently and fairly deal with him - you will.
Also, if you haven't done so - do a little research on ADHD so that you know what behaviors are brought on by ADHD. Punishment for those behaviors should be done differently. Also realize that his attitude has also been shaped by years of ADHD and (probably) adults and teachers yelling at him for his behavior. Consequently, he literally has to unlearn some behavior-isms he has picked up over the last several years.
Hope this helps. Good Luck!!
I might suggest you read my answer to "teababy - Difficult Behavior" posted on Oct. 21 on this site. Hope that I am wrong ...