Fwiw, I have ADHD. If I had been diagnosed at 11 (instead of 43...) my life would have been way way better. So this is a good thing. Honest!
I think the first thing to get your head around is what type of ADHD and how does it affect him. It's a bit like ASD in that it has 3 basic strands (inattentive, hyperactive, impulsive) and kids can have one or two or all three and to differing extents. Plus each trait can affect the individual child differently. So though there are stereotypical behaviours and issues, each child is different and when you are reading up about it, you pick and choose the things that are relevant to him and will help him.
The next thing is that this isn't an excuse, it's a reason. There's a difference. He's not going to get away with stuff and you're not going to let him. Because being a kid is all about learning to cope with life as an adult and life isn't going to cut him any slack just because he's got a diagnosis. So if he has impulsivity problems, that doesn't make it OK to shout out in class or deck people if they annoy him. What it does mean that you can stop blaming him for it - it may not be his fault but he needs to learn extra controls and coping strategies in a way his friends don't. One of the problems that comes with ADHD is low self-esteem, because you are often labelled "lazy", "rude", "aggressive" or whatever: you still need to work on the behaviour that gets that reaction, but it's easier to do that if you realise it's the condition's fault and those are not your real qualities iyswim.
You say "ADD" so I'm guessing he's primarily inattentive, like me. In which case he needs structure and heaps of it. You need to put a scaffolding of routine and reminders around his life; notes in his school bag and reminders by the front door and alarms on his phone and calendars on his PC and any and every type of system you can set up to support him.
It will be fine, honestly. ADHD has benefits as well as drawbacks; he'll be a fabulously innovative thinker with bags of enthusiasm and energy and focus for things that grab his interest. I actually found work a lot easier than education once I'd stumbled across the right career.