Your post is odd because it sounds like you think he just caught ADHD and didn't have it this whole time. The things you find daunting were daunting before, right? The new diagnosis gets him extra time (I assume) but the coping skills kind of come from him and you figuring out systems that work. At this point, in private school especially, you don't really get new things. Teachers weren't withholding anything.
Are you trying medication? That may help make this less daunting to him

Is there a learning specialist at the school who can help you navigate the school resources/practices? Some schools really believe in students taking the lead.

The diagnosis is new, the child is not. Whatever worked last year will continue to work this year. Is there something in particular that challenged him last year that you need help with?

In public at best you’d get teacher assistance with filling out a planner and you’d get a case management report weekly. If you’re really lucky the teachers will keep current with whatever online grade book system the school uses.

Teachers have lots of students, maybe 100s, and don’t have regular communication with parents. Also schools don’t tend to communicate with privately hired and retained people.

What did you do last year? Was it enough, not enough, too much? If you just got the diagnosis, does that mean that last year didn't go well? What would you have done differently then, in hindsight?

Hello - I have a rising junior just diagnosed w ADHD. I am just learning about resources and bc he is in private, have to mostly navigate/provide these things myself. The school offers some support, but not a true IEP or anything like that. He is going to have a therapist for sure, and I am considering EF coaching. However, what I find daunting is the day-to-day management of things like assignments and navigating this across 6-7 teachers/classes. I work a ton, the other parent has untreated ADHD, and siblings. Is there some type of professional that helps w teacher communication and keeping track of assignments w the student, etc.?