I do understand it being nice to have someone put a name to what you are working with. That part may mean you can obtain some services (insurance needs a diagnosis to justify paying for things). As Keista said however, it really is simply a name that says he displays certain behaviors and characteristics. Many kids could fall under that "diagnosis" including kids with bipolar disorder, mood disorders, learning disabilities, autism, developmental delays...etc.... Until you know that there is no other umbrella diagnosis or what conditions are adding up to make him not be able to do well....it really is hard to get true help.
It seems you already have found the truth, traditional behavioral methods without appropriate therapies and out of the box thinking really are not effective. The blame then falls on parents often times (and as you say, we can all use help but really....unless you are sleeping thru this, sounds like you have done enough things that he would likely not struggle this much if he COULD do better.)
So that is the bad and the GOOD news because there is really more help out there.
First, an easy thing you can do to start to get some immediate help is to read books like The Explosive Child and Lost in School by Ross Greene, and What Your Explosive Child is Trying to Tell You by Doug Riley (sp?.someday I will remember how to spell his name reily? riely, riley?? )
These books will help you to see why what you have tried is probably not working and will give very real ideas to help.
Next it sounds like, if you have not yet done this, a more complete evaluation would be helpful. IF this has gone on since he was a toddler it is likely he is wired differently. Something is going on inside of him that can't process the world around him as typical kids can. He may seem selfish and defiant but it is likely he can't take another person's perspective without being taught that skill. It is just how it is for some, and even then for many it just doesn't happen to a normal degree. Then as people are frustrated with them their identity becomes negative about themselves...so it becomes a vicious cycle.
Sounds like his intellectual ability is good. He may have specific strengths and weaknesses in learning style or many other issues.
A plan you might consider is to have a complete Occupational Therapist (OT) (occupational therapy) evaluation to check for subtle fine motor and for sensory integration challenges. (does he ever seek or avoid tastes, smells, sounds, sights, textures, including limiting food types or covering his ears etc...) Also a complete Speech/Language Pathology evaluation to see about his ability to process sounds/language. IF they suspect an issue they may recommend an auditory processing evaluation (not a hearing test, it shows how they are actually processing the sounds that they hear like whether they can hear the important sounds-i.e. a teachers voice/directions--when other sounds are around him.
Bring those results then to a complete evaluation with a neuropsychologist. a neuropsychologist evaluation is done by a specialized psychologist who has additional training in neurology and can help figure out how behavior is connected to the way our brains work. They can give a more complete diagnosis comparing whether there is a mood disorder or a developmental disorder etc. They can help you understand how his brain interprets the world. This then can help you find specific interventions or whether or not you might need a consult for medication etc.
Psychologists and psychiatrists who work in general practices tend to focus on psychiatric diagnoses. Ed. psychs tend to look more for learning issues, etc... you get the idea. It is nice to go to a place where they have a broader view.
I am sorry for his and your challenges. You will see that this is a group full of people who have lived through the feeling that we could be doing better, then the feeling that we are doing well but the therapy community at large does not really get kids who are wired differently.
Hang in there, it is a long process but well worth the effort when you find more specific answers.
WELCOME again! Dee