My first pregnancy was very difficult and when my miracle baby was born at 32 weeks, I thought that was it for me. Her first year was difficult because we had to deal with a lot of preemie issues. She was sick all the time (she had asthma too) and she was developmentally delayed. We spent a lot of time at doctor's offices and getting her the therapies she needed. Things got much better after the first year and I decided that, in the future, I would probably want another child. My husband felt very differently. He was happy with the child we had and didn't want to take any chances of things not "working out" the next time around. We argued about it and then I just let it drop.
When our oldest was nearing her third birthday, she asked if she would ever have a brother or sister because she wanted one. That opened to door to revisit the conversation about having more children. After many long discussions, we decided that we didn't want to deny her the opportunity of growing up with siblings.
Two months before her fourth birthday, my daughter got her wish-a baby sister. Six months later, I was pregnant again which was a great surprise. Shortly after her fifth birthday, she had a baby brother too.
Your baby is still young, it is probably hard for your husband to envision him three years old, walking, potty trained, dressing himself, etc... . Unless you are ready to get pregnant now, I would suggest you drop the topic until you are sure you are ready to start trying again. He may just be overwhelmed with all the new responsibilities that come with a baby.