I have noticed that in the last year he has been able to write his numbers smaller and a little faster, a little more legibly, but I think because of the hypotonia his hands might always get tired a little faster than most people's. So I think it might be a reasonable accommodation to allow him to do math his way.
If you can get him in to the developmental pediatrician this may be a good thing to ask about. What are reasonable long term goals for handwriting for him? While it is true that hypotonia may mean he gets more tired, it may be something he can learn to work with especially if he gets on a plan of increasing his muscular strength and has plenty of practice to improve those neural connections.
While I'm guessing most of us can see the problem with timed tests or putting kids in elementary under pressure to write out tons of problems, I would be cautious about generalizing from that to simply deciding it is an accommodation not to require written work for middle or high school. Even kids who are very gifted in math generally find once the work is appropriately challenging they need to write out work in order to solve problems. If he plans to do high school math or go to college he will certainly need to write out answers at that point.
I remember you mentioned there is a problem getting OT. In case you aren't already familiar with it I wanted to mention the therabands and theraputty. Both are pretty inexpensive and easy to work with at home. They made a HUGE difference for our hypotonic chid's endurance and hand strength. A few links:
http://library.osfhealthcare.org/PatientEd/Rehabpdf/OTDocuments/NEWOT/Theraputty.pdfhttp://www.ncpad.org/exercise/fact_sheet.php?sheet=259&view=allhttp://www.ncpad.org/exercise/fact_sheet.php?sheet=258&view=all