This is tricky, first off he may not feel confident enough to take that step. It's a really big one. Coming from I guy I say you need to make him feel like things are perfect and theres only one thing left to do. if he's sure he'll do

you can always kind of hint at it, talk more about how you two belong together forever, but i wouldn't suggest aproching him directly, this aggressiveness might scare him into making a decision without thinking (IE not to marry)

My husband said he knew he was ready to propose to me when he realized I wasnt desperate for it like other women tend to be and was happy with how our life together is now, not some childish fairy tale. Just Back Off! It shouldn't be forced