Oppositional Defiant Disorder (ODD) Support Group
Oppositional Defiant Disorder (ODD) an ongoing pattern of disobedient, hostile, and defiant behavior toward authority figures that goes beyond the bounds of normal childhood behavior. When a child cannot seem to control his anger or frustration, even over what seems to be trivial or simple to others, the child will often react in violent or negative ways to his own...

Sounds like your son might have even been trying to find a common ground to strike up a conversation with him, just probably chose the wrong time to do it. You said something to him about it and gave him some good advice. Other than that, I would let it go unless the school comes back with something else that is more serious and or concrete.
I would actually commend him for holding himself together during the whole thing. That's what I would be focusing on here. How your son handled himself in the situation. Let him know you are proud of him for that and that the administrators pointed it out and were happy with his response.