Here is the test (and be serious about it, if he senses you are joking and not serious, it allows him to continue manipulating & controlling you the way he currently does)
You are willing to indulge in his 'polyamorous' (seriously a stupid word) ways if he is willing to indulge in you allowing the same thing in your life.
Tell him that for most of your life, you thought that it was all you need was one person but you are kind of seeing it from his perspective now and think you may want to try it. You met someone at work (or at school or through a friend, whatever, just make it believable) and you notice there is chemistry between you & this other man but never pursued it because you wanted to be true to your mate.
But now that you are starting to see how it can be an open experience for your partner to love more than one person that you are thinking you may want to try it out too just to see how it feels, possibly you could enjoy it as much as your boyfriend currently enjoys it.
Check out his expression, if he is fine with it, look into pursuing another relationship with another man. If he isn't fine with it (which I'll just assume he isn't because I think I know better), he will question why you need to do this. He may even get a little angry or jealous that you brought up this new thinking.
In life 3 things are sure: we love ourselves more than anything else (regardless of what we are willing to admit), we want things we can't or don't have and we don't want to be controlled or manipulated or restricted.
Your boyfriend loves himself very much (and obviously much more than he loves you), he wants to have his cake (in this case pie, pardon the pun) and he wants to eat pie from different bakeries without being tied to eating pie at just one bakery and he doesn't want you to control or restrict him.
He has the best of all worlds: he has you, he knows your devoted to him and attached (at the hip almost). He has you and has you very easily, which is why he doesn't want you as much and wants others. When we have something already, we don't want it as much, that's just life and how it works.
You want him to want you & only you?
Detach yourself from him. Stop making him your world.
Show him that you can be with other people, spend more time with friends.
Go out, start to make yourself look better, shopping, tanning, makeovers, clothes, going to the gym, getting dressed & looking hot when you go out, start dating other men, etc.
See the reaction you get from him. I'm sure it will change the tune he's whistling. He will no longer have control over you, he will lose his power over you and you will become very attractive to him, so much so that he will forget his other ambitions (polyamorous).
Either way, if your partner doesn't want to be with you exclusively, drop him.
If you really want to keep him, show him what he'll be losing. Remove yourself from his grip, make yourself more attractive to other people, start dating, enjoying your life without him. If it doesn't phase him at all, you are losing nothing, and if it does bother him, wake him up and tell him that you are nobody's slave or fool
Respect yourself, you deserve much better than this man is giving you.
If you've read my posts in other discussions, I'm all about equal treatment for both men & women. I'm not just about men getting fair treatment, I'm about all of us getting fair treatment & respect. I hate it when either side tries to take advantage of the other.
Don't let him take advantage of your love anymore.
Respect yourself and teach this boy a lesson!
I'd be interested to hear how this turns out if you are willing to try my suggestion, I have an idea as to what will happen but I'd love to hear about it from you. Let us know!!!