If he wants his own children, he'll have to end the relationship and meet someone else.
Simple.
You two could try without contraception but it'll be extremely unlikely you fall pregnant, if you do it will be extremely unlikely that the pregnancy doesn't end in miscarriage.
I'm not sure that you need the physical and mental effects of a miscarriage while trying to be a Mum to three kids under 10.
You could probably only have kids together by using a donor egg and doing IVF.
This is expensive.
It would be very hard on you.
The baby would not be your biological child.
The fact that 50 yr old sperm was used would still be a risk for all the things that are being established as higher risk for older fathers
.
It is probably not in the best interests of your children.
Why is he even raising this with a late 40s woman ....does he not understand basic biology? It's only outliers (women) who have babies in their late 40s onward.
Does he have learning difficulties?
He can't imagine or process potential problems with any baby born from people in their late 40s and 50??
He sounds like he has "issues".
Not unlike the issues that led him to delay having kids because of money til 50 (and counting).
Loads of people have children when not in perfect financial circumstances. Who would have any if their circumstances were not "perfect".
If you really really wanted kids, you would compromise around that