I would get a shit hot solicitor.
If you still want to marry him.
You have to negotiate division of assets you build together, what if the majority of childcare falls on you, sick days, picking up from school, whilst he is happily building up his untouchable business?
Do you both want children?
He should keep what he builds up perhaps before children but what afterwards?
He's right you should get a shit hot lawyer.
It's very different if you start a marriage on equal footing, want the same things, build up everything together, and you know you need to get married for protection, he also knows he could lose more than is sustainable or even worth it for him.
If you have children they need protecting. You need to know that if you do divorce, because this is what this is all about, that it won't just be the basic child maintenance, that he will fund their schooling, and they will be entitled to assets you both build up.
This is sensible, practical, everyone knows where they stand financially, and hopefully you can come out with a legally bound document that is fair and protects you both and any subsequent children.
And if you aren't happy with anything you can walk away, as can he. I am not sure he is fully taking into account the full extent of what you can include in this document but he will find out in time.
Isn't it better if you do get married, that you aren't thinking, he might screw me over here and he isn't thinking, fuck I might lose my business on your wedding day?