My situation is similar.
I am with a girl who has a child from another relationship.
When I met her I hadnt considered children at all. But, because I love her so much the fact that she has a child didnt put me off, I wanted to be with her so embraced family life and we are doing really well.
It is an interesting dynamic, the way we all regard each other and admitedly the situation forms the basis of some rows in the house. For example - I am too strict with the child and I dont read his behaviour as a parent would or could etc....
She keeps saying "when we have our own you will realise...."
But I am not in that place yet, we have discussed it at length and I am so glad we can be so honest with each other. I know she wants a second child but I know that being together is more important to us. Perhaps one day I will want a child and maybe so too will your man. I am a similar age to your guy, never expected to be a father, if at all, until I am mid 30s