The best and worst thing you could do to this sleezo is call up the local KH during a Sunday morning and ask to speak to an elder about a very serious matter.
When you get an elder on the phone tell him it's regarding so-and-so, give the elder the name and ask if he's a member in that congregation and if not ask how you can contact an elder in his congregation.
When you get an elder of the right congregation you tell him that you'd been having a sexual relationship with this man and that you've found out you weren't the only one and you're upset about his selfish and opportunistic way of using women for sex and dumping them later.
Tell them you are well aware, as are others, that he is a JW and he's comporting himself in this way. Ask him to look into this and that you would like to follow up with this elder in two to four weeks time.