A: Why not? First, he does not want to be singled out as different, second, he will get some test-taking practice and you get some information on how he deals with such tests. Third, achievement test results are mailed to parents, not posted on a bulletin board or made public in any way. Fourth, if you feel that test results are not representative of your child's achievements because the child came to the U.S. from a foreign country, you may have the test score removed from the child's record by filling out a form which you aren't told about unless you ask.
The trouble with standardized achievement tests is that, while the tests are standardized, the kids are not.