Belle,
My sympathy to you and your entire family.
Your montessori sounds very like the one my son was in for 1st and 2nd grade that turned into a nightmare. My only suggestion is if it works for your family please try to get him out of that environment as soon as possible. Although he wants to be in a class, maybe homeschooling would work better with a couple of class style activities thrown into your schedule?
If it is any help, alot of DS's (age 9)SPD issues have inproved greatly now and he is reasonable happy in the public schools for now
Hugs to you all
