Do you need to be married? It isn't a big deal these days.
Perhaps not to you, but it is to many people in society.
OP, I don't think you should do anything like book the registry and tell him to show up. I wouldn't be impressed if that was done to me.
He should marry you of his own free will.
I'll be honest with you. Having children is one of the reasons that many people get married to the one they love.
You've had children already, so from his POV, there's not much incentive to get married, especially having seen what happened with his parents, he knows if anything happens he can walk away from you easier and with much less of a financial hit.
I'd just say to anyone who wants to get married, don't have children first, if marriage is very important to you.
I got married after 2 years of being in a relationship. My DH was not moving at a pace I was happy with and I told him that I wanted to take a break (I was ready to end the relationship,) so he had space to think about it. As I wasn't prepared to waste my time.
I was able to do that because I didn't have children to think about.
He didn't want a break,but
I had a timeline in my mind and would have walked if nothing changed. As it happens he proposed within that time.... and we've had 18 years as a married couple.