If only life were so simple, that we could offer "THE" solution to problems as complex as yours...
This Should Be Discussed BEFORE You Get Married
I don't want to get off on a rant here, but... Marriage is a serious commitment all too often entered into too lightly by one or both parties. Major issues like having and raising children should certainly be discussed before the vows are taken, but communication about this and other topics is very commonly neglected by couples caught up in the passion and excitement of love (at any age).
This said, your situation is by no means hopeless or as grim as it may seem.
Generally speaking, you and your husband should make an effort to really talk about things that are important to you individually and as a couple, in order to avoid potentially serious conflicts in the future.
Specifically relating to this issue, a possible solution to present to your husband would be adoption. I fully understand his reticence to become a new parent (again) at or after age 50, especially to a newborn-infant-toddler, who will require many years of doting care.
Why Not Adopt?
An option you should consider is adopting an older child - there are hundreds, probably thousands, of children of all ages, from grade school through adolescence, who need loving homes and families. I would suggest you contact your local child/family social services office (look that up in the government section of the phone book) and/or an adoption agency (they'll be in your yellow pages).
I wish you and your family the best in your new life adventure!