I have gone with the "different people learn different things at different rates" approach. Just like some people are faster runners than others. We have also talked about how being better at X doesn't mean the same thing as being a better person. It just means that doing X takes more practice for some people. You can give an example of something that your child finds difficult.
I have tried to avoid the "smart" label since it seems to imply that others are "dumb". It's difficult though, because casual acquaintances notice DS and tell him that he's "really smart" all the time.
