I wanted to get involved in this thread, because I find it really interesting. I really, really don't want to rain on your parade, because you sound so adorable!
I can see problems here, sweetie!
I think I can see what is happening. You were in distress at the thought of being poly (understandable). BF was distressed about hurting you (understandable). Both of you backed down and tried to change your viewpoints (admirable). You were elated when he said he'd try to be mono (understandable). Because you felt respected by his loving act, your brain is now more at ease - the fact that he is willing to do that for you is wonderful. It should give you more security. So, now that your subconscious mind has more security, it is able to let go of the reigns a bit - hence thinking about 'freebies'. You feel more important, more valued, so you naturally are more able to let go.
However... when you became elated, what your brain immediately heard was "I will be monogamous". To appease you, he said he'd "try". He didn't say "I will be". Your rational mind recognises this, because you're telling yourself that you will be ok *when* he slips up. You and he both know that he may very well sleep with someone else.
It's not good to have a relationship structure that is set on failing. Trust me; cheating is much harder to deal with than poly. In poly, the main things you have to battle is: thought of loved-one with someone else (and all the offshoots of that). When someone cheats, even if you expected it, the things you have to battle are: thought of loved-one with someone else (and all the offshoots of that), knowledge that your loved one is capable of hiding something from you, knowledge that they are capable of doing something that will hurt you, knowledge that they are capable of breaking your trust. Cheating is much, much harder to get over.
You could say to him "ok, have three freebies a year - in fact, you don't even have to tell me about it, as long as they are one-offs". What does this actually achieve? Well, it creates a wedge of secrecy between you, for one. It might also drive you slowly insane. You'll text him one night and he won't reply until the following morning. You'll be thinking "is he with someone else? Is this one of those freebies?" As for him, it's likely that he'll feel free in the moment, but battle with guilt afterwards. That isn't a healthy situation to be in.
You could have a 'don't ask don't tell' relationship; where he is free to do as he wants. I've considered this myself, because details drive me crazy. But I find that what I gain in peace, I lose in intimacy with my girlfriend. When I don't know what's going on in her life, I no longer feel like her confidant, her best friend.
I do think there are some merits in your freebie idea... but I think that actually, the merits lie in the idea of setting some boundaries. I think it's ok to start poly with stricter boundaries than you may eventually end up with. As long as they are realistic ones. Realistic would be "I don't think I could deal with you falling in love with someone else". Unrealistic would be "do not fall in love with someone else". Realistic would be "I don't want you to stay overnight with someone else / I don't want you to fluid bond with anyone / I don't want you to spend more time sleeping with other people than you do with me".
If he truly does need poly (not 'to sleep with other people' - but *needs* the *freedom* to have the option) and you truly need monogamy (not to control him - but to be in a relationship where there are no others), but neither of you want to leave, there has to be a considered compromise. Don't rush it, take your time. Discuss and debate and negotiate. Never do something purely for the other - it's a lovely concept, but it doesn't work. There are certain things that we can do, purely for the happiness of our loved one, but being in a poly relationship in *general* just for a loved one gets exhausting. You have to be able to find something that you genuinely believe is good for you, too - like, equal freedom, personal growth, or whatever else might be a good thing about poly.
Maybe a good starting point would be to go back to him for another talk, asking what his motivation is for being poly. What is it that he needs? Or wants? Why? Understanding each other is important. I do still like the idea that other people have suggested, about having a trial period for either mono or poly. If mono, cheating is not an option. If you are going to be mono, you have to really, truly be mono. You could try that for three months. Or, you could try a very structured way of easing yourselves into poly. What about online play, like adult chatrooms or Utherverse/Second Life? Or that he can kiss guys in bars when he's out at night, but can't take it further? Or another arrangement that both of you prefer the sound of?
Me: (30f) open poly
GF: (40f) My long-term, long-distance partner
Hubby (37m): GF's husband
Garcon (26m): GF's submissive/third partner
“Peace comes from within. Do not seek it without." ~ Buddha