There's a summer camp program in my area (Baltimore) called Camp Invention. I don't know a lot about it, but one of my son's friends attended a couple of weeks and said he really enjoyed it. If I remember correctly they build different stuff each week.
Another option would be to get your hands on some scrap wood and let him loose with a hammer, nails and wood glue. Throw in some craft supplies like googly eyes, pompoms and pipe cleaners and see what he creates.
We are almost 4 months into having our newly adopted 7 year old in ourMy son is also severly ADHD, impulsive, and hyper type. He has loved building since he was old enough to use his fingers. He never "played" with toys, he always was trying to figure out how to take them apart, put them together, fix them, etc. My mother was the one who noticed it was unusual behavior, she knew something was up. With him being my first child, I did not.
We have after school programs that teach how to build model airplanes, and then fly them, motorized robots, etc. There has to be something in your area. Call one of the trade schools and ask them, they may not offer it, but may know who does.
We have had success with a lego camp that is in to the technic robots - they design and program them. My son ialso likes computers. Maybe computers is an area of interest? What about chess? I am just trying to think of other areas that your son may be interested in, in addition to the building.
My son also LOVES trains. He loves to redesign track layouts over and over again, that is another way your son could build, if he is interested. DS also loves puzzles, always has, even puzzle books - searching for things.
What about sports, maybe baseball - he can still have his own space but play the sport. Just a thought besides the building and taking apart.
I have a feeling your DS is very bright, just bored sometimes. You will find the right hobby for him, just keep calling around and asking. He is lucky to have you!