Op, you've had a tough time here, you didn't ask for your relationship to be picked apart, just what to do about broaching the subject of marriage (although, I do add PP have brought up good points about inheritance etc)
We haven't got the answers for you, Op.
Marriage is what you want, you have to ask. Doesn't have to be a huge thing.
Sat down having dinner, a simple, weve been together X amount of time, I would really like to get married, what are you thoughts?
...on he has the answer. Be prepared for a no. Especially if he's protecting his assets. If its a no, ask why. Would you both be willing to sign paperwork to protect each others interests?
Unfortunately marriage is viewed as all romance and big days and happily ever after, reality is there's alot of legal shit behind it too.
You need to discuss it with DP and be prepared for marriage to be off the cards tbh, you cannot sit around waiting for DP to hopefully ask, he likely won't.
Please let us know what you intend to do!