He should have simply given the usual long-winded response that politicians always give. The kid was a smartass, and should have identified himself. I do not consider this to be assault or battery, since no one was harmed physically. I do not see where this congressman had any obligation to answer any questions, just as people who are NOT congressmen have no obligation to answer any questions to unidentified pests.
His apology could not hurt, so he is probably right to have given it.