ADD is not a single issue, but a collection of issues with varied symptoms and coping methods, and which he is likely experiencing to different degrees of severity.
Thus, in my 20 years of experience coping with it, I've found the concept of ADHD to an unhelpful abstraction. Push past it and learn about all of the individual underlying issues and related concepts (attention and focus, memory (paying special attention to working memory), capacity for organization and planning, mental flexibility, emotional regulation, impulse control). Figure out how badly these individual issues affect him (they make workbooks for this purpose), then work consistently and patiently to help him understand them and to build up toolkit of coping mechanisms.
If he goes on medication, I would encourage you to investigate appropriate dosing and ensure that he starts out as low as possible so as to minimize the negative impacts. You would expect doctors to do this as a matter of course, but in my experience they do not. Don’t let some jackass turn your nephew into a zombie by proscribing 50mg of Vyvanse when 5mg of Ritalin could be both more beneficial and cause less of a change on his personality.
> you would expect doctors to do this as a matter of course, but in my experience they do not.
My experiences match this as well. After my diagnosis from a dubious battery of psychometrics and interviews from a psychologist, I was expecting the actual psychiatric care to be, I don't know, more scientific?
However, I was met with just simple brute-forcing medications and dosages. No measurements of any kind were ever taken. I'd just get asked questions like, "How do you like <medication>?" or "How is <medication> working for you?" Depending on my answers, dosages were either adjusted or some other medication was thrown at the wall to see what would stick.
Asking questions like, "How do I know if <medication/dosage> is truly working as intended" or "What should I expect to change in my life from taking <medication/dosage>?" were questions that I was never received a satisfactory answer.
I quit going to psychs a fews year back and only go to my GP now. Honestly, the care has been substantially better and somewhat more holistic too. I can actually see what good/bad effects medication(s) have on my body (blood pressure, heart rate, etc.), which various psychs, never measured a single time in almost a decade of care.
A little more on this topic. No teenager with ADHD will be interested in doing the work I've described above. If his parents aren't interested in it either, your ability to help will be severely limited.
One way to help him to do this work will be to get him out of his intellectual comfort zone. Encourage him to take on tough but fun projects (building big models, electronics kits, doing NAND to Tetris, etc) that will require foresight and planning. Make yourself available if he gets stuck, so that you can help explain that hard problems are only hard if you lack the mental framework required to parse and subdivide and organize them into simpler sub-tasks. If you can get him to sit back and think about how to solve hard problems, you'll be giving him a transferable skill which will be integral to working through life with ADHD and minimizing its impact--and you can do it without actually broaching the topic.