Unless you have reason to believe he has learnt this behaviour from being smacked by someone as a punishment this seems like normal toddler behaviour. They will often find a way to gain our attention when frustrated. Ignoring the smacking is probably your best option, react only to his frustration by empathising that its 'not fair' whilst ensuring no means no so he knows where he stands.

Thank you for reply.

It is clear that he has learned that slapping is a form of punishment. Initially he started by slap others but when told that he can not slap elders, he has started slapping himself.

You are right, he knows that this act make everyone pay attention to him. We will now ignore his this behaviour.

Thanks again.