I don't see this as a situation where the partner is being unsupportive.
His reasons for not continuing with the pregnancy are very rational and are also based on compassion regarding the impact of having another baby on the OP and their DD.
Unfortunately, being supportive doesn't always mean agreeing with someone or enabling poor decisions.
It's an awful situation for the OP and my heart goes out to her.
It's a very difficult decision to make, even if your head knows it's the best decision for yourself and your family.
I'm very much a believer that the woman has the final say in such a situation, but ignoring the consequences and expecting everything will be ok is unwise.
Just as women pressured into termination can suffer for years after with guilt and (potential relationship ending levels of) resentment the same is also true in reverse for a partner pressured into accepting an unwanted pregnancy.
As pp's have said, feeling and wishes don't pay the bills or support the pressure on a partner trying to hold a family together with their wife potentially being very ill post partum , two young children (one with additional needs) and hold down a job to keep the finances coming in.
There is truth in the saying it's never the right time to have a baby, but the fact is that there are also some times/circumstances when it's a really bad idea to have a baby and sadly in this case the OP's partner makes some very valid points.