Great advice above! There are several threads about this question, but unfortunately they're not readily coming up in my searching, so I'll throw in a few more thoughts.
As parents, we tend to fear labels. I think the most important thing I've learned is that our kids already have labels. Your son has undoubtedly labelled himself, his schoolmates have labelled him, his teachers have labelled him. "Defiant" is probably the nicest of those; I would suspect the labels he's selected in his own head are far worse. Stupid, bad, and unlikable are pretty common, and were certainly on my kids' lists. With a diagnosis, you are replacing those destructive and inaccurate labels with labels that are both correct, and provide a pathway to effective support. Labels can be an extraordinarily empowering and positive thing when they lead to better support, and show a child that they are not a "bad kid".
I started our first conversation with, "We finally understand why school has been such a struggle for you: you have a brain that works differently from most of the kids in your class. When we teach you differently in a way that matches the way you learn, you learn really well." (In DD's case (ADHD + dyslexic), we told her it's because she was a demigod, and she was thrilled - if your son hasn't read Rick Riordan yet, I highly recommend him.) As aeh noted, we talk a lot about how there are some things you can do far more easily than other people, and some things which are much harder. There's neither credit nor blame attached to this, it's just neurobiology.
You're not responsible for how your brain works, but you are responsible for understanding those strengths and weaknesses, and making good choices for how to manage them. Especially with ADHD, I've found it really crucial to try and help my son understand that it is not his fault that he can't absorb oral instruction and is always late. He is not to blame. But - - - he must own these challenges. It is his responsibility to build (with help!) the back-up systems and scaffolds (alarms, lists, arranging reminders, taking photos of classroom boards, etc) that replace the functions his brain doesn't do naturally, and to learn how to recognize the impact that his behaviour - especially chronic lateness - has on other people. Blame is a heave load to carry through life, and removing (at least some of) it is incredibly freeing.
You have the benefit of already seeing what a difference the right environment makes, so your son can see how different it is for him when his learning needs are being better met. I've also spent a lot of time re-iterating the apocryphal Einstein quote, "If you judge a fish by how well it climbs a tree, it will spend its whole life thinking it's stupid." My DS is most definitely a fish in a school system designed for monkeys, and sometimes needs to be reminded that he's doing pretty amazing things for a fish, and oh by the way, do you remember what you did in that stream last weekend?
The odds are your DS has developed a pretty negative self-view, so look on the diagnosis is a chance to slowly start to replace and rebuild with a more accurate and helpful picture. ADHD is neither good nor bad, it just is. The more you can talk about it in an open, everyday, matter-of-fact way, and show how happy you are to have this insight, and make the explicit link to how you are using this new knowledge to better support him, the more he can hopefully respond to his diagnosis the same way. Without stigma, without shame, just with better self-knowledge and better ideas how to move forward. Sending tons of good wishes to you!
