Jen, just a thought: are you sure that Montessori is the best place for your DS?
I ask because curriculum and delivery style matters, and yet it's not always as one might expect things to turn out. My tendency as a parent is all toward Montessori/informal schooling; and yet I have a DS8 who really does best in a rulesy, old-fashioned, worksheety kind of school environment. Montessori would not have worked for him.
An advantage of the more old-fashioned style of delivering content is that they have more cut-and-dried ideas about what constitutes mastery than Montessori schools tend to, so acceleration is a bit easier for them to gauge and accept. (You can do this worksheet? OK, move on to a harder worksheet.) Not saying there aren't disadvantages of this style, there are plenty, just saying that sometimes it is important to match the style to the kid's pattern of strengths and weaknesses.
I know you're in the middle of many considerations and I don't mean to muddy the waters for you... just thinking sympathetically of you and your DS who sounds a great deal like my DS at that age, complete with meltdowns and inattention, ability to complete preferred vs. non-preferred work, and so forth.
I agree with you: a neuropsych will be able to tune your understanding of 2E issues in a way that an educational psych may or may not be able to.
DeeDee
