I wanted to get this strand up from the bottom of the list when I responded. I’ve been avidly reading the responses to your questions and I may have something to add for you.
My daughter has a similar profile as your son and is entering 4th grade in the fall. In third grade, she attended a special day class in a public school in a neighboring district. As a component of the program, she was also assigned to a mainstream third grade class where she went for the less academic areas, such as art, music, PE, and some science and social studies units. I felt it was a good mix. She got the academics she needed in an appropriate setting, yet was afforded the opportunity to be part of the regular ed program as well.
Next year, the special day class will be eliminated due to numbers. However, she will attend the resource class for math and reading instruction and will have an aide in the regular ed 4th grade class for the other academic areas, such as science and social studies. Perhaps your school has a similar program?
Regarding your view on ADHD, I’m sure someone far more knowledgeable than I am will contribute here, but it is my understanding that kids with ADD (unlike ADHD - the H being hyperactive) do not necessarily show signs of hyperactivity, just distractibility. I wouldn’t rule it out just on that. However, I can tell you that my daughter can also be inattentive. I feel that she is inattentive when she is unable to comprehend or process what’s going on which, in her case, happens more often than with “normal” kids.
I hope my contribution has been helpful to you. Good luck with your son.