ADHD teens tend to be rather oppositional. Personally, I always had a really hard time accepting unsolicited advice from anybody and that includes loved ones.
The best thing somebody with ADHD can do is externalize everything they want/need to remember. Preferably in something centralized and always accessible like a notebook or planner app. Then make it a routine to read through and take care of the stuff in it daily.
Watching that video helped me put my behaviors in perspective and really understand how my brain works. He even says in the video that “chaining a notebook to yourself” is the best thing you can do if you have ADHD. If your son believes that he has ADHD that video, or others like it, might help him understand what that means and he’ll probably start to draw the correct conclusions.
I wish you and your son nothing but the best. Growing up with undiagnosed ADHD was both the best and worst experience of my life. The highs are high and lows are so unfortunately low.
We actually did a full neuro psych evaluation when he was in 4th grade and that was eye opening. It let me "see" how his brain was working, and, for my wife, opened her realize that she probably has undiagnosed ADHD. My son and wife were constantly butting heads and the eval helped my wife understand and reason better, too.
Be gentle, but consistent and always outline how routine makes his/her life better. For instance, lay out your desk a certain way and you'll finish your work earlier and have more time to spend gaming. Acknowledging productive tasks that allow for dealing with pent up energy is also important.
My parents for some reason thought hanging negatives over my head always mentioning "think of what would go wrong if you don't do the right thing". Honestly, this just left me with a bunch of hugely negative self-talk issues that took a while to unpack since many of those things also had to do with my ADHD.
Thanks for the reply and sharing your own experience. We've definitely noticed positive reinforcement is HUUUGE. He's so proud of himself when he remembers to get to bed on time, or completes the morning routing w/out reminders.