Now that my twins are 6.5 yrs old, they don't elicit as many spontaneous "Oh, he is SO smart" comments, but when they were younger, I would usually say something like "He is really interested in learning," or "he likes to read," or "he loves (subject X). I think that type of comment hit a peak at age 4 - once they turned five and acted "older" their comments just weren't as shocking to others, as they didn't appear so young.