I was in a one room school for kindergarten and the first grade and was then in a larger school with 3rd and 4th together and then 5th and 6th together. I loved listening to the teacher teach the older children after I had finished my lessons (or while working on them) and loved hearing the reviews she gave the younger ones. I am not easily distracted though and do not have ADHD. I was never in a homogeneous classroom.
My children have been in both and I think there is much to be said for children helping others who may be struggling a bit and to have them become compassionate over those with different skills and deficits. I am now a great grandmother and my husband and I love volunteering as tutors in excellent schools where there is also a teachers aide every day helping the teacher and we see the children pairing off with a "reading partner". There is a lot more noise and activity than would have been tolerated in my day when the only one speaking was the teacher, but the children seem to be able to pay attention to their won project and ignore the surrounding confusion.
I would recommend helping the teacher in the class as much as you might be able, and enlisting his dad as well. You can see for your self if it works for your son!! Sounds like the teacher and the school are both sensitive to the child's needs.
blessings!!
N.