It is both normal and wise for him to give serious thought and concern to anything that might have a significant impact on his children.
In nuclear families, parents will.consider the question of having yet, another child if having an additional child will! In any way effect the quality of life that they can provide for their existing children. Sometimes, it becomes a question of is this the right time to have another child? It may be a questions of how many children can we afford to send to college , do we plan to send them to private schools, etc?
There are many questions that parents struggle with in making the decision to increase the family size. Your partner is just doing what any caring or responsible parent would be doing. He is trying to decide how the impact of this decision will effect his life going forward.
When we divorced, my ex and I agreed that neither of us would have more children. We were each positioned to provide the quality of life that we wanted for our existing children and this was the size of the family that we each wanted. During our divorce proceedings, he had a vasectomy. In addition to only wanting to 2 children, neither of us had ever been in a family where our siblings were born outside of our nuclear families, and we wanted the same structure for our children.
For us this was the right decision; we would not have chosen to have another child should we have stayed together.. Prior to getting married, we talked extensively about the number of children that we each wanted to have. Fortunately for us we were in agreement as to the number.
Your partner has to look at how this decision will impact both you and his existing circumstances. It sounds as though he is doing the caring and responsible thing.
You too should be considering his ability to both emotionally and financially support and provide for 3 or more children. There are so very many things to consider before making the decision to have a child.
Giving adequate thought to the decision can help to minimize problems later. A thoughtful man is the kind of father that you should want for your child.