You can probably only pull this off once, but have you tried saying "Well ... we weren't going to tell you this until you were older, son. But you're not black. You're actually ... Korean." (or whatever other ethnicity looks amusing, though he probably wouldn't get "Polish"), and maintaining a straight face for the crucial two or three seconds until his inner "WHAAAAAAT?" is complete?

--Dave, this one was not listed in Cruel Tricks To Play On Small Children