Shari,
I would tend to wait until he shows some kind of interest in this topic. My DS, for example, once told me (from his booster seat in the backseat of the car) that the other children in his class 'don't have a little voice.' He's the kind of kid that one really is forced to talk to about giftedness at age 6 - but the problem is similar to the 'Santa Claus isn't real' problem - it's really too irresistible for someone with age-appropriate maturity to keep to himself.
You can ask questions, though, about why he thinks that he goes to different classes for different subjects, and do other kids do this? The answer is usually that 'the principle' or some other 'Athority' feels that he will learn best this way.
Best Wishes,
Grinity
