I think it depends a bit on what your discussions have been like and where you are in terms of age/life stages etc
If you’re thinking in terms of getting married by x age, having one DC within x years, another by the time you’re x age etc then he may not be on the same page and not see any urgency to get married.
For a man, it’s a pretty straightforward ‘do I feel ready to wholly commit and not be able to shag anyone else forever?’ type scenario, but for women it is a more complex and more time sensitive game.
Many women have waited around to be ‘asked’ only to find their fertile years are behind them, their BF decides he doesn’t want to commit, they split and he marries and impregnates the next woman to come along.
Please don’t sit around patiently waiting as some are suggesting if you’re factoring kids into the equation. This is your life and you’re right to want some say in how/when you move onto the next stage.
I would suggest not making it a traditional ‘down on one knee’ proposal, as that may feel emasculating to a man who was planning to do that himself. You want an equal relationship so make it an equal conversation, not one where one of you makes the decision and the other either goes alone with it or crushes the other ones romantic dream!
I’m in a similar position. We’ve talked about it, both agreed it will happen one day etc (DP even asked me one night, kneeling by the side of the bed! But we subsequently split up and then got back together so I don’t think it still counts 
.)
I’ve been having a think about places I might like to get married and I thought about taking him there for afternoon tea one day and just saying “what about this place to get married?” and then take it from there.
If you’ve already discussed it as an idea and he seems in agreement, you can tell him you don’t want the big gesture or an expensive ring, you just want to make your relationship official.