I've found this to be one of the most challenging areas of dealing with a fine motor disability - our ds has not had any trouble at all showing his understanding of conceptual math on achievement tests, but timed fluency tests are beyond impossible to score highly in percentile on due to his difficulties with handwriting. The WJ-III Tests of Achievement are a mix of oral and written response, but only the three tests labeled "fluency" (reading, writing and math) are timed, so those are the tests that are *most* impacted by his disability. (Note: the subtests that require written responses but aren't timed are also impacted significantly enough that they don't represent his full knowledge by a long shot). Our neuropsych has told us that there are two forms of the WJ-III Achievement Tests (a form "a" and form "b") and that one way to gauge his knowledge on the fluency tests or other written response tests would be to give him the alternate form using oral response in place of written response. You could ask Davidson if they would accept that type of accommodation, given the documentation you have from your ds' OT evaluations.
The alternative is to submit a portfolio rather than achievement testing, which I suspect you could do since you are homeschooling and he's working 2+ years ahead of grade level in math.
Last note, if you ever want to know for your own information where your child is at, I second the suggestion to use the ALEKS testing - it's tied directly to each state's curriculum, and we've found it was one of the most useful pieces of data we had when advocating for our ds' subject acceleration in math. The assessment is all done on the computer, and it's not timed.
Good luck!
polarbear
