Evalluna, firstly you both sound like a lovely couple. You are being practical as regards finances and the effect it might have on your relationship, but you are both thinking about the potential of another DC in the right way, ie not merely as an inconvenience, but as a challenge because it is your third and trying to think through all the consequences of whether or not to go ahead with the baby. You are weighing up the options to make sure you make the right decision for your family, and each other. At such an emotional time, I commend your strength of character.
I am interested that, although you feel having the baby could put your relationship at risk, your DP actually sounds supportive at this time, although you alone can tell whether you feel this could put you on shakey ground over the longer term. However I do have a caveat about DPs level of support which I explain below.
I cant predict the future for you, or say with any certainty whether having the baby could work for you. It certainly isnt a financial impossibility but clearly the chilcare costs are high which sounds like your main concern. My response is that you do still need a "positive confirmation" from your DP that he feels fully committed and will not only support you through the pregnancy, but long term as well. Rather than assume what he feels, get him to honestly voice his concerns, get it all out in the open now. If you feel confident that he is resolute to support you, then you are both taking on the commitment of your third child together.
From what you describe, he has deferred the decision about "termination or not" to you and is stopping short of saying "I love you and want this baby together". You need him to take joint ownership and Sometimes its that last bit of commitment that makes the person shift their mental mindset into one where they too are investing emotionally, not standing on the sidelines!
Time is clearly of the essence so why not have that open dialogue soon. It takes two to tango, and he needs to be part of the dance!
Btw - It made me smile that your DP did some research on foetal development - as an academic, that is very much in character! Again, that gives the sense this is something he is thinking about deeply, not just making a knee- jerk reaction. You just need a bit more from him, to feel confident, I would suggest.