Tell your son

 

"You are not a racist.  Someone calling you a racist does not make you one.  Not everything is about race.  If someone thinks everything is about race, he is a racist by definition."

Too much time has passed, no need to stir it up again.  Playground verbal exchanges should be taken with context.  Your being alarmed would cause more problem for your son than what the teacher said.

Let your son learn a quote from Ben Franklin: "Remember not only to say the right thing in the right place, but far more difficult still, to leave unsaid the wrong thing at the tempting moment."