The fact that he has suddenly started saying this suggests that he has recently reached a new level of understanding about the differences between him and his age-peers. A comment like this could reflect his exploration of this difference, and perhaps an attempt by him to grapple with feeling incongruent.
The next time you hear this, there may be value in casually asking him why he said it. (Obviously, not in front of the recipient of the comment!) That might give you some insight into the direction you should go in discussing the social appropriateness of the comment.
We have frequent small conversations with our offspring about where the value of a human being lies, the diversity of giftings, appreciating others without artificially downplaying oneself, and how being blessed with any exceptional gift brings with it the proportional responsibility not only to use it for good, but also to be gracious to those less-gifted in that area.
