I want another baby, but my partner doesn’t

This problem is one of the most difficult that any couple faces and that's because, unlike most things where some give and take often diffuses the issue, you can't half have a child. 

I'm sure you're both right too. Many men and women crave another child and feel incomplete without one but face a partner who is adamant that having another baby will just not work for them. There are lots of reasons. Money, time effort and energy, mental and physical health limitations as well as commitment to the relationship can all be factors that influence people's stance.

From your letter, I'm very clear on your respective positions but you say nothing about the reason for your husband's view. I think he's harsh to accuse you of being manipulative just because you've expressed how you feel however, I wonder if he feels completely overwhelmed and his response is a way of hoping to close the conversation down. Clearly this hasn't and won't work so another conversation needs to be found that is more helpful and helps each of you to better hear the other. 

I completely understand that  the strength of your yearning for another child has thrown your relationship's viability into very sharp focus. There is so much to consider isn't there and it's not for me to sway you one way or the other. I would just say though that leaving to find someone with whom to have another child or choosing to have a baby without a partner will create a complex dynamic for your current family. Of course you must consider how or if you can get your need met but at the risk of offering an uncomfortable challenge, have you considered that even if you leave, it's by no means certain you would conceive a child and how co parenting and the impact on your little boy might work if you and your husband are not able to amicably draw your married relationship to a close. 

Ultimately, this is a time old problem. In my experience of counselling many couples who could not agree on whether to have a child it's very difficult to identify areas of common ground when each feels so vehemently that they are right and indeed, have a right to take the position they do. Sometimes though new understanding and appreciation of the other's position makes way for different conversations and potential acceptance, one way or the other. The desire to have child is so powerful and there is nothing wrong with that. Just make sure that in the midst of feeling something so profound, you don't ignore or choose not to see anything other than your heart's own desire and make decisions that could have life long implications for all concerned.