I am just going to throw a few things out here for you to look into, I would have no idea which ones, if any could help you, but having kids with these kinds of issues that aged through childhood, we did see changes at certain points.
Puberty is rough for kids like ours. What worked for them at 5-10 quit working when they hit the puberty zone, everything changed from their behaviro, to what kind of meds they needed to get through. The stimulants caused agitation, and we had to adjust, they needed different and very closely monitored antidepressent meds and something to help them deal with mood. They just could not deal with the normal flashes and dips in thier growing and raging little bodies, so my best advice is to find a very attentive psychiatrist, maybe one that takes no insurance at all, because they might be able to see him as often as he needs it. This was an expensive process for us, but worth every penny.
Another thing I would look at is an educational evaluation. If he has never been evaluated by the school, ask for it, in writing. Some kids with ADHD are very bright, but have some basic and fundamental learning issues associated with processing, but because they are so bright, they can manage the lower grades with shear force. It is easier for a moody 12 year old to lie and argue than it is to tell adults that he has no idea why he can't keep up anymore. It is almost never the case that he is just lazy or wants to be in trouble, that is a common presentation and he is frustrated and needs your help.
When they reach middle school age, they just can't pull it off anymore because of basic processing issues like processing speed, working memeory or visual processing skills and many measures of poor executive function. Get both a school evaluation and a private evaluation by a nuerupsychologis (as your psychiatrist for a referal) and I would request something called a Woodcock Cognative from your private evaluator (not all you need, but a really good tool to show you how he processes information.)
Go to www.wrightslaw.com and do some reading about how to advocate with the school for good services for your son. A child whose grades are dropping and has ADHD warrents evaluation and assistance. He does not have to fail before he gets help. I would also suggest that you read "The Myth of Laziness" by Dr. Mel Levine. It just is not the case that kids want this presentation, he is probably more misserable than you are, for sure!
Last, if the psychiatrist did not explain ODD, ask again, and if you have a good one, they will take the time to explain everything you need to know about what they see and how they are treating your son.
M.