Since you are not marry yet, suggest you give up as you already at 下風, even he give up the new one, you can't gurantee there will have another one. He may request you to give him more money or accept another one as he know you can't live without him!
Both me and my sister face the same problem before. She give up that guy and finally she married another one, now she have two kids and she is quite happy.
I'm not that good, I found my ex-husband have another one just 1 week before marry, I still marry him as we already book the restaurant and everyone know I'm going to marry, finally he still leave me. Anyway, I also married another guy later on, my bigger son is 6 years old and younger son is 3 years old already.
Believe my experience, hope you good luck.
You can call 明愛 to ask for advise, too.