Your son's teacher should have a very good idea of whether or not he is able to focus on tasks or is easily distracted. It is quite possible that the environment in the public school is less structured than what he was accustomed to at the religious private school. The children may be able to move around and talk to each other more, causing your son to be distracted.
Parents are usually good judges of their children's abilities. You also need to understand that test results are not completely reliable indicators of how smart young children are. So many things can cause children to do poorly on tests. In any case, a single test score should never be the only factor in making placement decisions.
It's time for a talk with your son's teacher about how well he is really doing in school. Ask to look at the work of both average and superior students to see how it compares to your son's work. Then discuss ways that you and the teacher can work together to ensure that your son has the best possible classroom experiences. Do remember that you have a responsibility for seeing that your son has enriching educational experiences at home.