The diagnosis is not a complete surprise but it is hard on him. He has always had problems concentrating at school, for instance he is good at math but would often misread the problems and answer the wrong question.
Do you have advice for him to be able to go through his teenage years?
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.