Collected advice from a text request.
I wouldn’t say there’s a way to cure it, but there are basic health things plus coping strategies that make it a lot better. Health things: The easiest and best thing he can do is go paleo, basically meat, vegetables, and fish oil. The biggest gain I ever made was when I ate a pound of beef per day for a couple years in a row, the difference in mental state was uncanny.
Exercise is huge. If he were doing brutal workouts twice or three times a day he wouldn’t have the energy to be scatterbrained. The stress from a chaotic, fast-life-history style environment also plays a big part, so removing serious sources of depression helps a lot. Sleep obviously, that has to be the first thing fixed if it’s not in place.
Coping strategies are more idiosyncratic and IQ-dependent, and they have to be goal-directed. The way it usually works is the guy decides he wants something in particular, then he needs focus to get it, then he figures out his own idiosyncratic hacks and copes to the best of his ability.
But…all of that aside, if I were designing a treatment program, it would look like the Primitive Technology YouTube activities, or the military’s SERE training. The primary problem an ADHD person faces in modern life is a lack of immediate, concrete feedback on their work (which reinforces the skill of focus) and there’s nothing more concrete than constructing a shelter with stone tools you made yourself. The key is building up that association between goal-directed action and the tangible result of it. Like with porn, it’s about reinforcing the pursuit of the thing, not the thing itself.
Once he’s got the concrete association down where he can play in the mud and be happy (I should mention being unhappy drives a lot of ADHD) try to get him into Brian Tracy’s goal-setting method. But a minimal level of focus is a prerequisite before doing that.
Aeoli Pera 
This tracks with my experience with ADHD.
Fish oil is a must, I felt the brain fog lift for the first time in 15 years palpably after a month of taking 50 kCal a day. other vitamin mixes for energy, calm, and even one called “neurolink” will help too.
I now run a groundclearing crew. a lot of the time, it’s axes-shovels-picks. you excercise with concrete feedback for money. the stress makes it a marginal tradeoff but you’d have a lot less stress working instead of bossing.
depression has a lot to do with frustration, hiring an executive assistant or having a gf who can help you remember things is key for staying on track long term.
Brian Tracey’s Ultimate Goals Program is what I use annually. I might have even got the idea from AP. I’m sure it’d be better to review every month or week but planning is painful.
Trying to fit in with your peers is rough but it can be done under the above circumstances. The people who I’m closest to are all from scouts or church- hands on, immediate feedback activities.
With ADHD, I’ve noticed that waking up late and sleeping late drastically negatively impacts productivity for me. However, I’ll have a tendency to daydream and hyperfocus on whatever (But without it being actually productive, most of the time). On the contrary sleeping and waking up really early (as in, sleeping at 5 P:M and waking up between midnight and 2 A:M) makes me productive in a way that is impossible to attain otherwise. It’s as if my brain had two modes that were switched on and off depending on when I wake up.