Excellent question, although I am sorry to hear you have had such struggles with your kiddo. Let me hit your questions one at a time:
(1) to be evaluated for ADHD (and bipolar disorder - it is obviously impossible to diagnose over the internet, but these are the two most likely culprits if you feel the issue is organic as well as behavioral) you must take your son to either the pediatrician, or (better yet) to a therapist or psychologist/psychiatrist who specializes in treating children. There are quick-n-dirty evaluation checklists and instruments on-line, and while they may give you some sense of a direction, they are not completely valid for a formal diagnosis. Only a doc who can sit hands-on and face-to-face can initiate the diagnostic process. If medication is warranted - and it sounds as if a medication trial will probably be started - then a medical doc has to be involved.
(2) ODD does not really have a formal evaluation. It is not a neurochemical issue, but rather is a behavioral issue. If your child reacts in overtly defiant ways and seems to like having something to "push" against, then it is likely that this diagnosis will be assigned. And if you Google "Oppositional Defiant Disorder," you will find the diagnostic criteria and the symptoms required to earn that particular diagnosis.
(3) As for a behavioral plan, I recommend a therapist who can more comprehensively assess your family, the strategies you have already tried, and the reasons for success or lack of success. In general however, we try to move parents from a "consequence avoidance" system of behavioral management ("Do it, or else") and toward a "goal attainment" system (positive reinforcement; "You control your fate because if you are complaint, then you earn..... privileges"). I know you have done a fair amount of this already (every parent tries everything they can think of!), which is why a local therapist is in the best position to give suggestions tailored to your unique situation.
Sorry to be evasive on that last part, but there are too many variables to give a really comprehensive behavioral management plan over the internet and without knowing you well. If you are satisfied with the response, please hit "Accept." That is the only way I can receive credit for my answer. Thanks-