Here's a post I put up to a similar question a few months back...
As an ADHD adult I can tell you that this is nothing to fool around with. Here's some startling statistics for you...
1.) The number one cause of death of ADHD teens and adults is fatal car accidents
2.) The average ADHD kid/teen/adult is approx 100x more likely to visit the ER than their non-ADHD peers
This is ENTIRELY due to not paying attention, and it is almost entirely avoidable.
I see it myself, and today is a perfect example:
My ADHD meds seem to increase my potential for being seasick, so I will often skip them if I'm going boat diving. Was a bit choppy this AM so I didn't take my meds.
First dive, I could feel I wasn't "on top of everything" so I was double careful to check all my gear and configuration, etc. They set the hook, and we all suit up. I have my buddy check me again and I'm all set. Jersey roll into the water and I'm FREEZING immediately!
Dammit, I must have a leaking seal on my drysuit. Down the anchor line like a STONE, which is odd, because with my saltwater set-up and drysuit I'm usually pulling myself down the line. More air in the suit, but still feeling squeeze, still feeling cold, still very negative. Except for my feet which are ballooning, which isn't usually a problem.
Short dive because I figure I need to go up and tend to a leaky neck seal.
Get on the boat, doff the suit and I can feel that my fleece is still BONE dry!.
That's odd, no?
Then I also think: Why/how is it that I'm wearing my cheepo fleece OVER my Bare 300g undergarments?
OOPS!!!!
Seems I'm not WEARING my 300g thinsulate bodysuit, just the thin pair of cheepo nylon fleece I usually wear UNDER my body suit.
Obviously I got distracted while gearing up and didn't put them on.
What was I doing instead? Chatting with the other divers, cracking jokes, discussing gear configs and other dives, etc.
No harm, no foul; right?
This time. Suppose next time it was a BCD inflator hose, or my tank valve, or my dive computer, or my pony bottle?
Scary stuff - and I KNOW I have the problem and do/did everything I can to ensure I'm on top of stuff.
People can think what they want about ADHD, but telling someone with the disorder to "just focus" is like telling someone with asthma to "just breathe" or telling someone with poor eyesight to "just look."
Am I saying he shouldn't dive? No, but to the extent that his condition is not adequately controlled, you'll have to think about answering the question on the medical form as to whether he "has a medical condition that would prevent him from being a safe diver."