Oh N.! 5th Grade is so much fun too! I would have for your son to miss out on the great curriculum because he doesn't like to write. I was a 5th grade teacher until I had kids and now I am "retired" to be home with them, so from my point of view, here are some suggestions:
There are kids who just DON'T like to write. I have come across plenty of them- you ask for a paragraph and they give you a "yes or no" answer.. literally.
A few things I would look at: you say he is able to verbally express himself well, but won't write... (is this paragraphs, full sentences, will he circle a multiple choice question, do math problems?) Is he able to organize paragraphs?
One thing I found with a lot of my students is that they could express to me what they wanted to say but when it came time to put it on paper, they couldn't organize themselves. I would start with letting him answer questions with a graphic organizer- begin with one word answers, then build up to fragments, full sentences, then full paragraphs. (If you don't have any graphic organizers, let me know and I will email you some samples if you are interested).
Also, how does he feel about typing? Would his teachers allow him to type his homework that requires written responses? Obviously it is not ideal for standardized testing (in fact I am not sure if it is allowed with the MCAS unless they are special ed, which I don't even know what those regulations are anymore they have changed it so much...)
When he does write, how is his actual writing? Is he required to use cursive or is he allowed to print still?
I wish you all the best- you sound like very supportive parents and I am sure it will all work out..