Be wary if teachers discount your concerns based on standardized test scores. Some children can do pretty well on tests, but the scores don't reflect day-to-day performance. (We know the opposite is true, too.) "Average" on standardized tests often means: "compared to all other kids in America," and not "compared to those wiz-kids sitting on either side of your child, who, by the way, are almost a year older than he, and who's parents are Nobel prize winners, and who are in the gifted and talented program at the museum of science." The point is, your son may be a wonderful kid with average skills and average potential sitting in a class in which he's essentially a minority. The advantages of being in such an environment are many: expectations are high, language is rich, learning challenges abound, and under the right conditions your son benefits from all of this. However, if teachers don't see this as a potential problem, your son can compare himself to his classmates, and never quite feel that he's making the grade.
It's also possible that there are impediments to your son's learning (although the average Iowa test scores argue against it). You should have the child study team (also called the Teacher Assistance Team or the Teacher Support Team, or something like that) take a look at your child. This team of teachers and other professionals has the responsibility of determining if children may be in need of special education services, or if they will do better if the classroom is modified in some way. This team may suggest a comprehensive evaluation, or you may request that yourself. Be careful about how test results are interpreted, though. In some school districts, an average IQ or achievement test score is the standard for normalcy -- it's what's expected of most kids. Unfortunately, in other areas it's a cause for concern or even, sadly, a mark of shame. It may turn out that there are no learning disabilities or other impediments to learning. If you don't feel that your son's teachers can individualize in a way that will make your son feel good about himself without standing out too much, then you may want to consider another class (or school) in which the students exhibit a wider range of academic and potential.