P.M.
If I were standing in your boyfriend's shoes, I can see that he might think you are planning out his life for him without his input. Of course the best time to talk over family size would be before starting a pregnancy, so you can determine whether you are well-matched as mates. In this case, that step apparently didn't happen, so you'll both have to work harder as reassessing your futures.
Providing for children is a huge undertaking even in a flourishing economy. You don't say how long you've been together, but it sounds like he is not the father of 'your' first child. It's rather heroic for him to step in as the dad, if that is what has occurred already. It might be a good thing to count your blessings, and leave talk of additional children for a future, less stressful time. You've both got plenty to deal with right now.
Also, as I have already done twice this evening to other requests regarding family size, I'd like to beg you to consider the impact of human population on the planet that is sustaining us all. The earth's carrying capacity is already seriously strained, and young parents would do well to consider the potential impact of this on the children they are bringing into the world. What we "want" is not always what we truly need.