The two things that jump out at me are:
a) that he said he would drop her tomorrow if you would go mono with him
b) that you don't like him having another girlfriend, even though you have another boyfriend
Regarding (a) that's a red flag to me. Even though this is your thread and it should be about you, I have to say that's kind of a jerky thing to do to this girl. She's probably expecting a relationship, but he's just using her to keep distracted when you're out. It would be more appropriate for him to find some other friends or a hobby to keep busy with when you're out, rather than taking advantage of that girl.
(b) is basically your issue and you need to overcome it. Fine, your boyfriend thought he was mono and so you figured you would never have to deal with jealousy etc. Well, surprise! He's not feeling so mono anymore, and you have to go through the same shit you're asking him to go through. That is assuming, of course, that he truly feels poly and isn't just using her, see (a).
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."