Please do not uninvite the boy, and please do not let him or anyone else find out the invitation was an accident--that would be hurtful and terribly poor behaviour.
WHile I'm sure your son is upset and feels the other boy was "mean" to him, most likely the other boy feels your son was "mean". They are little kids--that's how it goes. Teach your son to be a gracious host and to be the first to try to mend a relationship--as others have said, they may never be best friends....but then again, they might in a couple of years. They're still young and sorting things out. At any rate, it will certainly be a good way to teach your son how to 'rise above'.
And definitely keep an eye on the party for any signs of exclusion or 'mean boy' behaviour. None of our kids are perfect.
