You definitely don't want to pressure him. It will only result in resentment later
By asking him he could see it as the only reason he's marrying you is because you are pregnant. He probably doesn't want people to see that as why y'all want to get married. I saw your post earlier. I've been with mine for 4yrs. We have a 3yr old and I'm 5 months. We decided to wait til 2016 to get married. (I'll be 21 at that point). I'm sure he wants to wait til the timing is right.
Mayb marriage scares him? Or he wants to surprise you and not have someone tell him? I've seen it both ways or mayb he feels to young