I'm sorry to hear that your brother has been turned off by his childhood experience with the meds. I had a similar experience. I was on Ritalin from the time I was ~7 until I graduated from high school. When I graduated I thought "great! I don't have to take that shit anymore!"... and I proceeded to waste the next ~12 years of my life. Going back on meds at ~30 was completely transformative--like waking up from a bad dream.

Even if he won't take the meds, he could still benefit from one of the many ADHD workbooks that are out there. I was initially skeptical, but I finally bought one[0] and worked through it, and I found it to be really helpful. I had always thought of my ADHD as one problem (I just can't attention!), but the workbook breaks it down into distinct sub-problems, one of which (poor mental flexibility) I honestly hadn't even recognized as a problem until I read the book. For me, breaking down the problem has made it easier to deal with.

Another thing that might help him out is journaling. I am still occasionally able to go weeks without a conscious awareness that ADHD is wreaking havoc in my life--but not when I'm journaling. By setting aside a little time each night to A) reflect on my values and B) take stock of the day, the feedback loop goes from days/weeks down to 24 hours, which makes it much, much easier to prevent my disease from having a negative impact on me and my loved ones.

Sorry for all the unsolicited advice. You sound like a good brother, and I figured this might help. Godspeed.

[0]https://www.amazon.com/Thriving-Adult-ADHD-Strengthen-Functi...