You could ask him if he'd let you adopt a child. But adoptions run a high risk of being a poor temperament match between mother and child, and every adopted child has been abandoned by its mother so has a deep & unconscious wound. Age 40 is also a fairly high risk age for having a first child, and his sperm could increase that risk.
forming a marriage is a challenge to create a healthy balance between love and power; and the person who says NO is exerting more power--so that's HIM. You're paying a high price for delaying your acceptance of a man into marriage; and he's not wanting to pay a high price for 1. divorcing or being divorced by 2 prior women, and 2. "robbing the cradle" by asking you to marry him (and thus taking advantage of his (possibly) higher adult achievement and wealth than yours). You're also in different stages of your lives, even if you don't notice that at present. Because soon he will probably retire from his career in the world and be at home, possibly also wanting more time and adaptability from you than you'll be comfortable giving to him, ONCE THE 3 YEAR honeymood-stage of marriage has passed.
So as an initial response, I'm saying: "Look into the future of your time together with him,. Sketch out what you'll be doing in 3 years (when the romantic feelings subside somewhat, and you both need some other aspects of life you LOVE to keep you happy) in 6 years (when he might be ending his full-time work), in 10 years (he's retired, you're not?), in 15 years (his health could be declining, slowly or rapidly), in 20 years (what's that look like?). After you've imagined your life together WITHOUT a growing child of your own and WITH your interactions with HIS grown children, THEN start injecting YOUR little child into the scenes at 3 yrs (just born), 6 yrs (toddler, dad's interest determined by HIS other activities and comparisons to his past children), 10 yrs (etc.)
Then decide what you think and feel about those 2 alternative life-lines. And consider this: You HAVE made a significant choice at a late stage of your fertility: What are your options and chances if you try to undo it and start over.
I'm suggesting that you recruit your imagination (plus intuition) and your thinking (plus feeling) for evaluating your options. Not just your feeling and perhaps past imagination about having a child of your own. You could also ask other women your age and beyond for their evaluation. But they could confuse you if you haven't ALREADY done your own evaluation by using the steps I've suggested.