In addition to other good things you’re doing… another concept that sounds like it would really help your son (and most of the students I’ve taught) is something that isn’t unique to ADHD… it’s the concept of “growth” and “fixed” mindsets. Ned Hallowell said something to the effect that this concept is the most important contribution from psychology in a really long time (wish I could remember and quote it better). People can easily slip into feeling that “smart” or “stupid” are fixed traits of a person… something that you’re born with that there is no ability to change. So why work hard if this is just a fact about you… the harder you work the more it hurts to fail. If you know you didn’t try, it protects you a bit from the pain of failing. Anyway, I think teaching kids (and ourselves) to shift to a “growth mindset” (explained in the podcast below) is key to happiness in life and effectiveness in school. What you’re describing with your son sounds like bullies and other brain chemistry stuff are for sure playing a role in making it harder to try and more painful to fail…but the hopelessness and giving up that you describe sound like classic symptoms of a “fixed mindset”.
Here’s a great podcast from Ned Hallowell on this topic. They are discussing 3rd graders, but I have seen great change in the college students I teach with these same techniques – I’m sure it applies at any age.