This is a very difficult area for our family. My son,now 9, says that looking back at kindergarden he was "bored all the time but didn't know it," he seemed completely happy then, but by age 6 he was creating havoc at school. I have chosen to share with him that his tests show that he is at the 1/1000 type of unusuallness. (the good part is that they can understand the normal distribution curve at an early age - I'd work on that for a while first if possible) Rimm's books on parenting gifted children are good at explaining how not to "support" you child against adults innapropriatly. To a child, not fitting in means that they are "less than" no matter how good their deviation from the norm looks to an adult.
I'm so glad for your family that you are looking for options NOW, and not a year or two later after the situations is muddied by any behavioral reactions to "lack of fit" in the school setting.
Part of your explaination can be that part of what makes your son different is that he has a need to learn new things. While you are exploring options, don't hesitate to start meeting your child's need to learn new things at home with you. At our house we call it, afterschooling. Sitting down and learning together will demonstrate to your child that you see him for who he is, and that it's ok to be who he is. This is more important that any discription of "how smart/deviant" your son is. Ask your son what he wants to learn, then post back here if needed for more resources for how to get it to him. If he doesn't know, pick an area that you like. This forum will be a resource for you beyond your wildest dreams. Love - Trinity
