Women Are Marriage-Minded; Men Are Goal-Directed
What you have described is a fairly common scenario. Women tend to be fairly marriage minded and men tend to be rather goal directed. Of course these aren't absolutes, but it often works out that way.
What Makes A Man Commit?
So, what does it take to make a man commit? From the men that I have talked to (and research that has been done), men generally make decisions based on a benefit -loss system. In other words, "If I make a commitment and marry this girl, what will I gain and what will I lose?" This may sound cold and uncaring to the female sensitivities but it is just the way that the male psyche functions. So, the questions is, what would motivate your guy to propose?
At this point, I would say, not much.
Freedom Of Availability
The costs of marriage are high for a man (and a woman as well, but that's another subject). In getting married your guy would give up his freedom to date other women. Even if you two are monogamous (and I hope that you are), by not getting married, he retains the right to be available "if something better comes along."
Getting married also makes him financially responsible to you (and you to him, but he probably doesn't think that way). Men get high strung over this. I'm watching my fiancé do it right now.
Marriage also makes him emotionally responsible to you. Right now, he feels no obligation to travel with you or to take you out or to cultivate an interest in the things you enjoy. Why should he, this is just temporary.
Free Milk AND The Cow
Though you may think as "we", he is still thinking of "me." The things that a man would generally gain through marriage, a consistent lover and companion, someone to run the house (cooking, cleaning, etc.), this guy already has. Why in the world would he change that. I'm sorry to have to tell you that you have created your own problem. You have given him all of the benefits of marriage without requiring of him any of the responsibilities.
Male Logic v. Female Logic
I would bet that when you moved in with this guy, your expectation was that you would get married soon (i.e. within a year or so). Moving in together, from a female perspective, is the first stage of commitment but, for most women, it is not enough. We want to be married.
For a man, living together may well be enough. His logic may be something along the lines of, "Why mess with a good thing?"
I think that you already know what you need to do.
Don't just threaten and change your mind when he takes you out to a nice restaurant. Find an apartment of your own or move in with some friends. Tell him that if he really wants you as a wife, he will have to marry you.
What If He Does Propose?
Maybe he will propose. If he does, then you will need to determine your answer. Do you want to spend the rest of your life with THIS man or do you just want to be married? Think about it. It is better to hurt someone's feelings (even someone that you care deeply about) than it is to choose the wrong marriage partner.
Commitment Or Convenience
If you do decide to date other people, make it clear from the outset that you will not move in with a man until you are married. This simple statement will go a long way to weed out those men looking for a lifetime commitment, and those men who just want convenience.
Best of luck and stand firm,