Tell him private, at the end of the day.
Or suggest that the meeting beheld in a room with a big conference table he can sit at.
You can go ahead and cite the fact that these women likely come from conservative cultures, and the embarrassment can mean they don't sign up with your company. That affects the bottom line, and you can present it as a matter of marketing and diplomacy. Posture does convey a lot, and in the current climate, it makes perfect sense to take extra measures not to invite harassment suits.
It's a delicate subject, but in your case, this is affecting business more than will be apparent. He needs to be clued in that his casual posture is being talked about. That in itself will hopefully be enough for him to want to literally sit up and take notice. But do it in private, after everyone else has left, no witnesses to the conversation, and no emails or texts. And DO NOT speak to anyone else about it, ever. Even the women. Not a wink, not a nod. Guy's honor code- don't embarrass him.