If he does it to his current girlriend he will do it to you when someone more available,. younger, prettier, cleverer (any other adjective you like) comes along. You deserve to be with someone who is free to date. (unless you succeed in pulling this off when you need to remember Karma bites)
i dont mean to sound harsh, but he wont leave her, if he really wanted to, he would've done it already. he just wants he's bread buttered on both sides. besides, whats to say he wont do the same thing to you? (leave you for someone else that hes been having an afair with).
find yourself someone worthy of your time.
Keep in mind this mantra "if he'll sh*t on his girlfriend, he won't hesitate to sh*t on his 'bit on the side' from a greater height". Bin him, delete his number, and don't go rummaging around in the trash afterwards. The only way you're going to meet someone worthwhile is by clearing the decks, and your current guy is a non-starter.