You planned your first child based on an agreement between you both.
You now have a taste of what parenthood involves.
Although you knew 'you' wanted children young, did your partner share these wishes?
If you both agreed initially you didn't want this pregnancy you both should have made more of an effort to make sure it didn't happen. Now it is going to be a difficult decision either way. You have changed your mind (he hasn't) and with each passing day, you will bond more with the pregnancy and terminating more difficult.
You say you don't have the financial stability.... are you married, do you have joint finances or are they still kept separate? How long have you been together and were you financially secure when you met him or does your partner take on most of the financial burden of daily living. Understandably if this is the case you must understand why he says it will take a toll on his mental health. Can he cope, is he really as secure as you think he is, do you have debts, does he have any debts you may not know about.... so many things he could be dealing with without your knowledge. Maybe having another baby right now could push him over the edge. How secure is his job, any problems at work etc etc
If you continue the pregnancy you will have time off work, your income will go down, your expenses will go up. Can he absorb that and you still stay afloat not just living hand to mouth. He may resent you for forcing him to accept another addition. If you don't continue you may resent him and suffer guilt for a long time. Either way it could tear you apart. It depends just how strong your relationship is and what your support network is like.
You're right, partner doesn’t have a choice. He does have a choice based on your decision.
No one can tell you what the right thing is. But whatever your decision, it will be your life to live with the decision. A new baby could tear you apart, it depends on so many factors but equally it could bring you together if your relationship is strong enough.
It's a big decision either way.
On a personal note... I was that baby under similar circumstances but it was the other way round. Dad wanted another baby Mum didn't. I did not have a good childhood. I was fed, clothed and had a roof over my head but not loved. She resented me and constantly reminded how I ruined her life, my whole life (until I left home as soon as I could).