Try to comfort him and to calm him down. When you do calm him down then there is the hardest part to make him listen, and learn to handle his disagreement using words and nice gesture. Tell him that he should talk and say that hi likes something or he doesn't. With my older son we had a big success with talking and rewording him. Punish him didn't bring any good, because after being punished he did the same thing regardless.
Good luck.
Mirko
You are correct in your conclusion that slapping him will not be useful - in fact, it will likely serve to worsen the situation. Strong-willed children benefit from a structured, systematic behavior management approach. I can't go into detail here, but you can see perfect examples of what I recommend in Lynn Clark's book titled SOS: Help for Parents.