Everyone has said good stuff ... I particularly agree here with Ariakas.
When I met my current gf, I didn't think that I was into BDSM or kink and I didn't think I would be into polygamy either. Now I've realized that there are many facets to BDSM that I do enjoy and that I just had to give myself permission to admit that I fantasized about kinky stuff ... whether that was dominance/submission or masochism or bondage, or whatever. I am struggling with the non-monogamy right now ... but I'm learning a lot in this relationship and it has been the best relationship for me yet. So, it doesn't mean that it is a wash from the get go.
I would be careful about changing yourself JUST to be with him, however. There has to be something in it for you ... regardless of whether you were with him. Don't get me wrong, staying with my girlfriend because she is who she is, is important to me. But even if it was not for her, there are lessons here that are valuable for me to learn, regardless of any person that I am with. Does that make sense? If its really not for you, then its not for you. Don't make it all about him. But he could be presenting you with an opportunity here to grow within yourself.
Another point ... if you are not ready for him to be with someone else and you need some more time to process this, you can say that. You can ask for what you want. You can negotiate things with him. It doesn't have to be all or nothing. You can negotiate with him what he is and is not allowed to do with the other person, based on what you are comfortable with and not comfortable with at this time. The first time they are together, for example, you can ask that they not have sex. You can then see how you feel about the fact that they even spent time together. You can decide how much you want to know about their relationship. You can even negotiate around the frequency and length of time they spend together. If you've got a committed partner, then the possibilities are endless.
It is important that he acknowledges too, that you are new at this and that if you are going to give it a try, that he be willing to slow down a little bit to meet you a little more. I would hope that he doesn't feel like it has to be all or nothing. Thats just my opinion.