He is a sleazy letch with lax boundaries.
Putting himself at professional risk.
He will either listen to your concern, learn to respect you and behave professionally, or he won't - you will have to talk really seriously to him and gauge his reaction.
I would be asking him questions, and have a conversation, not just 'telling' him. I would ask him if he thinks women like being touched. I would ask him (neutrally, calmly) if he thinks it is OK for other men, your boss, perhaps, to slap your bum, put his hand there, or hold your hands. I would ask him if he thinks men and women generally have different expectations and boundaries. I would ask him if a boss or anyone with authority and influence over him has ever put their hand in an intimate place - and how would he react? I would ask him how he would feel if pictures of a man with his hand on your bum were all over social media.
This conversation might, of itself, wake him up to his behaviour, or it might tell you what you want to know.
Only you can decide what is the right threshold for you, and whether he is being honest about that threshold.