How should I bring up an open relationship?

I am in a friends with benefits relationship with someone I have been friends with for over a year. We have done so much for each other and have hung out on a daily basis in the past. Things are more sporadic now because we don't live as close anymore but we still see each other at least a couple of times a week. All our mutual friends thought he actually liked me for more than what he proposed, and they still think if he really wanted to he would ask me at least for a low-key relationship. He says he is against a serious relationship because of his busy schedule and since he may go abroad after our graduation from college (but that's not for another two years). I don't really want a full-blown serious commitment but I guess I am one of those people who views as a little more personal and would prefer an open relationship - that way we can both be free to explore our interests and go out with other people but we are each other's main interest. We already agreed to be ually monogamous. So at least that part is taken care of. I'm not sure how he would take to an open relationship - how should I bring this up and what do you think are his chances of agreeing?