Summer programs at local universities could possibly help him see that there are many other kids like him. Some programs do not require tests. My son was at an engineering camp last year, where they had kids from age 6 to 18 all working at their ability levels, alone or in small groups, but sharing their ideas with all. My son loved this type of environment. I think many kids go through the stage of trying to fit in, or feeling that they do not fit in. His teachers could provide you with some contextual info on what is going on in school that you may not hear directly from your son. I myself was one of those children who did not want to worry parents and often was hiding issues I had at school.
