Obviously, he's really comfortable in this relationship and he just figures, "why get married? I got all the benefits of a marriage without the legalities....my life is perfect."
So, if marriage is what you want from him, you need to tell him...FLAT OUT that you want to get married and give him a timeline. For men, things are black and white and you have to just say it. Don't assume he knows it. But, you need to be ready to leave if that's not what he wants, unless you're willing to stay in this relationship as is.
With my husband, after some time that I thought was appropriate, I just told him that I was ready to take it to the next level, but that if he wasn't, I understood and that it was okay, but that I was going to move on and date other people. I was matter of fact, not mad at all, not upset or crying, just matter of fact. After I decided to do that, he proposed shortly thereafter. If he wouldnt' have, I would've just moved on. I was so tired of men just keeping me lingering wondering if it would ever turn to anything, so I decided to take the matter into my own hands and it worked. We've been married for 8 years and have two wonderful children. He tells me now that if I wouldn't have done that, he probably would've lingered for awhile longer and that my telling him what I did made him make a decision. Of course, I didnt' have children so that may be different with you, but if marriage is what you want, tell him, be honest and be ready to move on. If he thinks you're bluffing with the moving on, he'll never propose if he thinks you'll stay regardless of what he does.
Good luck.