For my son who is now an adult, I read books by Dr. Doris Rapp (after everything else did not work). Yes, they can be very smart and very busy. As a good parent you have realized this and know he has a lot of trouble in doing anything different than being himself. Give him stimulating things to do, things to learn and lots of books to look at or read to him. Take him on "field trips" about things that intersest him and help him to have a very positive attitude about himself. Smile, a lot, and try not to take things too seriously. ADHD children don't always learn in the same way that other children do, and also don't like to bother with things that they don't think are worthy of learning. That can take some working through.
The thing that helped, tremendously, was reading the books, mentioned. In the books, you will find more than just typical adhd symptoms, but also some physical symptoms you may have noticed, as your son has been developing. Once you read these symptoms, you will understand that although medications may help some children, there is more going on than the parade of doctors care to deal with. One of the other responders mentioned a nutritional/foods approach and this, along with understanding that this is not the "fault" of the parent, helped greatly. Adjusting the diet, removing foods that the child is sensitive to, and just relaxing will help a lot. Look up: ADHD and candida; ADHD and leaky gut.
Always take time to talk with your son, and time to listen to him. Always keep a sense of humor. Always help him to appreciate his good qualities. Read and learn as much as you can about ADHD and about natural approaches to it. Look up the Youtube video with "Dr. Doris Rapp" as the header. Some possible foods that can cause problems: sugar, wheat, dairy, PEANUT PRODUCTS.