"he needs a good smack"
No, no he really doesn't. And not even because it's just not effective as a discipline, but because he doesn't react typically to either threats or actual pain. If threatened, with anything, he will earnestly question you on the precise nature and intent of the threat whilst showing no sign of fear or, indeed, compliance. This is my fault for saying things like "I'll throw you in the bin" or "I shall give you to the scrap man" when he was little.
And with pain, he will latch on like an enraged chimp and apply extreme pressure with his hands, arms and legs. This hurts, as I have found out whilst combing his knotty hair.
So although it doesn't look pretty, it is both kinder and more effective to let him howl it out until he is ready to listen, and this may not be for the rest of the day.
Like I say to interfering passerby .... You only have to walk past, I live with this and I know what I am doing.