He has valid concerns.
You do too.
Then there is, what about "custody?"
There are many kinds of 'child custody.' So research it.
Do you want another child, from him? Or from whomever? Meaning, is it just your biological clock, or you wanting a child with HIM? Only?
So discern that.
My friend and her husband, had a child when they were 46. But, by In-Vitro. It took several tries. But they now have their son, who is 5 years old.
There was also the concern about genetic problems. Of which, men and their age, can contribute to that.
Then, do you or he, have a solid retirement plan? MANY people, have to continue working way past retirement age nowadays. Despite the age of their children. This takes planning.
Then, will you both get married or not?
If not, then what if your relationship does not last? Then what?
That can happen with or without marriage.
The bottom-line is also: IS this the Man... you want to be with no matter what? IS he, a keeper? Or not?
It also seems, that you want to actually have a baby, and be pregnant. So, Adoption is not an option for you as you said.
But if you really want a child, with this man, then it should not matter. The point being, raising a child/a baby, even if adopted, as your/his child.
So it seems, you want to be pregnant. Not only having another child.
And how you will feel complete, if that does not happen.