I agree with Margy. Who knows what was happening - you need to ask first.
A child who is very consistently well behaved will probably continue to be so.
First of all get the facts from your son, his version may be differant, ask him waht happened do not judge until you have the facts , what was happening to make him a well behaved child suddenly lash out, did he hit first, what was said between them, if indeed it was all his fault talk to him about anger and hitting out at others , its up to you if you ground him if this is a one time I would speak to him firmly then let it go ..good luck