I don't know your boyfriend obviously, but my ex was very similar. When asked he would say that absolutely he did want to get married and have a family, but it was always some vague 'at some point' kind of answer. We got together at 21 so we were young but still, there wasn't even a real conversation about actual timescales, even when I made it clear that marriage was important to me, it was as if that didn't matter.
Things kind of came to a head and, although it wasn't an ultimatum really, I said we needed to decide where the relationship was actually going, so I could either give up on the idea of marriage or we could actually do it. Looking back I forced his hand really and he did propose. This was after about 9 years together.
We got married and the marriage lasted 18 months. I think what I really wanted was for him to grow up, and on some level I thought this would happen automatically if we got married. Safe to say it didn't.
I'm now married to someone else, and it was all so different. He proposed out of the blue after we had been together for two years, I just knew he wanted to marry me but I wasn't actually divorced for the first year we were together, or I think he would have proposed sooner. We have been married for seven years and have three children, it's all just fallen into place with no angst, because we have been on the same page throughout.
My point is, I suppose, to analyse why you need marriage so much and try to figure out why he doesn't. It's easier said than done, but what do you think will happen once you are married?