Hmm... I remember when I was homeschooled taking the Stanford tests every year (math and reading).
As far as a report of some sort goes, do you have any sort of set goals for the year (kind of like standards of learning that your state might follow)? Perhaps you could take a look and see if you've touched on any of the state standards and give him a mastered/not yet mastered mark by certain standards?
http://www.cde.ca.gov/be/st/ss/vagrade3.asp