As someone who had undiagnosed ADHD as a child, I want to say this.
It's admirable that you want to find ways without medicines. It is possible to learn tricks and workarounds, to an extent, and it is helpful to do so. You can help with that.
BUT it only goes a small part of the way
In ADHD, the brain, literally, can not shut stuff out properly. Can not self regulate attention and focus properly. Can not manage sleep cycles, time management, task switching, holding onto stuff when other stuff happens, starting and stopping things, properly.
This stuff is not "things to teach and learn". I can tell you from personal lived experience of the condition, you can explain it every day for the next 20 years, and he can understand it perfectly when you do. If he has ADHD, there is a good chance it will not make the slightest difference.
You may as well tell a person with paraplegia, every single day, what walking is like, and expect them to walk, as teach someone with ADHD how time and task management matter, or how not behaving impulsively is important sometimes, or to stay focused, and expect them to become skilled at it like others do. Or expect your car brakes to work if brake fluid is leaking.
(That's actually a pretty good analogy for how ADHD impacts, in the brain - stimulants are the "brake fluid" regulating focus, which is lost too quickly because of incorrect stimulant regulation; daily top-ups help with that).
The brain literally is not producing enough of the stimulant type of brain chemicals (neurotransmitters) for the circuits that handle that kind of function, to work properly. Even if he himself desperately wants it to.
Subjectively, think of ADHD a bit like being "locked in". You know what's needed... And it "somehow doesn't happen". Again, and again, and again, and again, and again. No matter how often you are told. No matter how often blamed, how bad you feel, what the consequences or incentives or punishments. Because the part of the brain that carries that stuff out, is not able to consistently or reliably do so.
Even when a person with ADHD does focus intensely - and they can - that's not a good sign. There's a state that people with ADHD call "hyperfocus". That's exactly the same problem. Brain can't change tack, disengage, or manage what it pays attention to or ignores. It's just as big a problem. Often it's more frustrating; it seems they can focus fine on things they like, so why not the rest. But it's just another ADHD trap. The brain has no real control over focus, whether that's lack of it, or excess of it. The only difference is you get more blamed and guilt tripped over the latter. Sometimes I couldn't do an essay. But sometimes I couldn't stop doing an essay, and nothing else got done for the next 3 days. Including basic self care, sleep, promised chores, and other homework. 1am... 5am... Brain is on a roller.
I have laid it on a bit heavily. One can learn a bit. But not well, or easily, or quickly, and often not enough to avoid trouble. You get very good at last minute, or rushing stuff or covering it up, or accepting the social and personal impacts, because what else can you do.
Even for something as simple as sleep cycle, as a kid, I've gone through behavioural, sleep hygiene, melatonin, motivational rewards, punishments/losses (in the form of things that don't happen if I'm not up), alarm clocks, and the result? Maybe one night in 8 or 10, I get to bed when it would be good for me, and the rest are anything from 8pm to 6am, to "not at all". Stack Exchange is one of the things my brain turns to, during those long hours. Because that's what it does, not necessarily always because that's what I know I need (meds help a lot but not everything's fixable even with them. I probably started on them too late. They help a lot with the worst symptoms however like "brain fog" and that alone is worth it). Homework at school? 8.30am on the bus. Social? Anything from 30 mins late for friends, to 2 hours, to total non show. Set an alarm? I'll hear it. It may or may not get me out of the door, or I might think of some random distraction as I head out and next thing it's 3 hours later and too late. Paperwork, forms, bills? You name it. And yes, over time tried pretty much everything. "Write notes"? Outcome, desk covered with notes, can't keep track. "Use a computer", outcome? Ended up on paper anyway, failed. Set a task or 2 a day? Some succeeded. Some took a week. Some didn't get done for a year or at all. Some took so much undue time and effort that a huge amount else was put off or never got done.
Then around 17 - 23 ish, it gets more fun. When people move out or go to college/university, and don't have family and support around every moment...... that was not fun times, in an ADHD sense. Some manage. And for some, it's when their tenuous hold on managing starts to slip. In small and private ways, that nobody sees, perhaps. That can be hidden. Or other ways.
What changed in the end? Meds. Not learning. Too late to salvage much of what I could have got at school, if I'd been on them sooner and hence more able to learn habits, and not be as I was.
ADHD is a nasty corrosive condition. This is what it's like.
So yes you can help and teach skills for it. That's highly recommended. I know that's your question. The truth is, they may work for some, but not that well, they don't fix it for real, and they never worked for me, and I don't know what would have, is the answer. I'm sorry. I live with this and none of the non-meds answers ever worked for me. So do not be so anti/reluctant if meds are recommended. They can be a life-changer. For real. That's what I would impress on you.