Have you and your DH actually gone in to observe the classes in the grade level into which your DS would skip? I'm not sure about your school, but in our district there is a huge range in student size in any grade, from kids who are young and big for grade to those who are old and small for grade. I get that you wouldn't want to put your DS into a situation where he would be at more risk for awkwardness than already necessary (i.e., give him yet another reason to stand out from the crowd). But you may find that even your very small boy might not be the smallest by much. Not saying it's not a valid concern, just that you might want to go observe before you make a decision.

She thought she could, so she did.