If he says, it’s nothing then ask him why he is doing so? Tell him very bluntly that you do not like this, and you want it to be stopped. You can talk to the girls involved, explaining that he is married, has one son and because of her marriage is in problem. You should be very friendly with girls involved (this is very important to awake woman inside her) and disclose his flirty nature. I’m sure any sensible girl will stop that.