Guys are often slow to pull the trigger. If I read your profile, you already have three children, and certainly that's an added factor to an already-huge commitment.
I know this seems radical, but have you talked to him about it? I mean, it's such a ridiculous taboo in our society. I'm divorced now, but 8 years into my marriage, in therapy, it came out that she had always been angry with me because I'd "taken too long to propose." Really? Why didn't she say anything at the time?
Anyway, just find a quiet time, sit him down, and tell him you'd really like to get married. You don't have to "propose" to him, but you could certainly get a sense of where he stands on it, maybe set some general timelines.
Isn't that sort of communication about difficult topics the hallmark of a truly good relationship? Why not start now?
~S