There may be a very good explanation of why your child performed so poorly this year. Perhaps he didn't feel well or didn't try to do his best on the test. Increasingly, students are required to take so many standardized tests that they don't always try as hard as they should. Before your son takes another achievement test, you might want to give him a pep talk about the advantages of trying to do well on these tests.
It would be a good idea for you to determine what the average composite test score was for other students in his grade this year. If the scores for most students have lowered dramatically from previous years, there may be a problem. Be concerned if most students are now scoring substantially below the 50th percentile in reading, language arts, and mathematics. Low scores in science and social studies do not necessarily indicate problems, because achievement tests don't always measure what the students were actually taught in these classes as curriculums vary from state to state.
If a substantial number of students in your son's grade didn't score as high as they have done in previous years, the problem needs to be addressed. Probably, the most effective way to do this is by having the parent-teacher organization work with the school administration.