I don’t know whether this will be helpful, but I’m an adult with ADD, and one thing that’s been consistent in my life is that I’m happiest and most motivated when I’m involved in something that matters in the big picture. Hobbies are great but, for me, usually fleeting. My motivation comes from the satisfaction of doing something that makes a difference. If there’s something your son has an interest in, you might create an opportunity for involvement, even in a small way, based on his existing skills.
I have a friend who, although fully adult, is much like your your son. Nothing ever seemed to motivate him. He’s morbidly obese, because even the threat of death isn’t enough to motivate him to move off the sofa. When I lost my driver for my snake rescue work last year, I got him to go out one night and drive for me. He wasn’t interested, but did it as a favor for me. That night, I picked up a gopher snake who was lying in the middle of the oncoming lane, and I carried him to the car for my friend to see before releasing him. While I released the snake, a car ran through the lane where I’d picked him up, and my friend made the connection — that specific snake, who he’d held in his hands and whose eyes he’d looked into would have been crushed to death by that car if we hadn’t been there. Because of that moment, the man who couldn’t be bothered to move off the sofa is now driving for me seven nights a week during rescue season, and has been gradually modifying his car — adding interior shelves for equipment and specialized lights that make it easier to spot small snakes on the road. Sometimes it just takes the right opportunity at the right time to create motivation.