I really like Sashaman's reply. I'll quote it, "I really like you and want to keep seeing you. What are your thoughts about dating exclusively and seeing where things go?" It is absolutely appropriate to bring it up after a few months. What's important about having it now, is if he says that he's not comfortable with that. That tells you where things stand. If you're okay with having an open relationship, then that'd work out fine. However, if you're not (I don't think you'd be asking the question if you were), then you need to start thinking about finding someone else who shares your core values.