You may want to consider the possibility that to him, your relationship isn't the "main" relationship... For some poly folk, there is a primary/secondary model (in which there would be a "main relationship"), but for others, there's no hierarchy. This might be something to discuss with him: "If you begin seeing other partners, how do you see our relationship?"
Given that it's only been two and a half months... I know from personal experience that love and commitment can grow rapidly. But in *general*, I would say that amount of time is barely enough to say you're "in a relationship", let alone that you're the "main" one. That may be a commitment he isn't able or willing to make so early on.
If you choose to accept him seeing other people, it's absolutely okay to feel hurt and insecure. ANY feelings are okay... it's the thoughts, words, and actions that you have in response to the feelings that matter. If you *feel* hurt and insecure, and use those feelings as reasons to pick fights with him or to doubt or interrogate him... that's when it's a problem. But if you feel hurt and insecure and say to him, "You know, I get the poly thing, but I'm feeling a little insecure about the idea of you seeing other people. Here are the things I'm concerned about: (list them)... Can you reassure me that those things aren't likely to happen?"
Own how you feel. Be mindful of how you phrase things; "You make me feel insecure when you talk about seeing others" puts the blame on your partner and is inaccurate. No one can MAKE you feel anything. Saying "When you talk about others, I feel insecure" might look the same on the surface, but it shows that you recognize and acknowledge that *you* are responsible for your own feelings; they might have arisen in *response* to his actions or words, but he is not to blame for them.
Insecurity and jealousy about a partner's poly-ness aren't exclusive to mono folks. I've dealt with it more than once since Hubby and I opened our marriage; it's one of the reasons Hubby has chosen not to see anyone else, though I've made it clear to him that I want him to make that choice for *his* reasons, not because of my feelings. I dealt with feeling insecure and jealous with the first boyfriend I had after I "came out" as poly, and I'm dealing with it now (as in literally at this moment) with S2, and it isn't the first time I've dealt with it with him.
But those feelings are *mine*... they aren't about anything the guys have said or done, they're about issues from my past, coupled with depression and anxiety disorder, whispering in the back of my mind that this can't possibly work and that if my guys start seeing someone else, they'll decide she's better than me and kick me to the curb, because I suck rocks. Because that's what I was taught to believe most of my life: I don't deserve good things or good people. And I know that my negative emotions are because of *that*, and so I can address them with the guys without being confrontational or blame-y.
It's okay to feel how you feel. If you can identify the root of those feelings and include that in your discussion with your partner, so much the better, because then he, too, will understand that *he* isn't doing anything wrong, you're struggling for your own reasons and are owning your reactions.
If you have strong feelings for this guy, before you decide whether or not to continue the relationship and whether or not to accept him seeing others, sit down with him. Lay it all out: All your fears, all your emotions on the subject, all your hopes. We're all strangers on the internet and can only give advice based on what you've told us; he knows you, cares for you, and is the one who can best answer your concerns.
