I don't agree for advices about the police.

A single slap, that nobody seemed to have notice ? It won't go far and just ruin his current job. He will be the one looked as a troublemaker for calling the police for a single slap.

Even if the manager can't do anything about it, ask to him, ask to your teammates about it. They probably know him enough to tell you how you should handle him. If it ever happen again retaliate, however not physically like @Kilisi said, SPEAK UP and LOUD enough without overdoing it : "A friendly slap you say ? You're nothing near of being my friend".

I don't agree for advices about the police.

A single slap, that nobody seemed to have notice ? It won't go far and just ruin his current job. He will be the one looked as a troublemaker for calling the police for a single slap.

Even if the manager can't do anything about it, ask to him, ask to your teammates about it. They probably know him enough to tell you how you should handle him. If it ever happen again retaliate, however not physically like @Kilisi said, SPEAK UP and LOUD enough without overdoing it : "A friendly slap you say ? You're nothing near of being my friend". Make sure to use strong words but no insults.