Financial parameters for step parenting

My partner and I have been together for 4 years and we are planning to get married this year. He has a 15 year old son who I’ve referred to as my step son for a long time, and we have a baby girl together who is 5 months old. Over the course of our relationship I have gradually taken on more and more financial responsibility for my step son. At first when we were dating, my partner would pay for them both for things. Over time we started splitting things 50/50. Then SS decided to live with us full time so 50/50 became more than it was before. Then once we had our baby girl we decided that since I have a dramatically higher income I would work and he would stay home with the kids. Now we have started talking about SS needing a car, paying for college, etc. I’m now getting SO overwhelmed and when we talk about it we just argue. My cost of living has multiplied by like 8 in the two years since we all moved in together and I am starting to resent the fact that we never talked about our financial expectations for step parenting. I’m not sure I think it’s appropriate to expect me to take on such significant financial expenses like a car and college. SS has a bio mom nearby but she can’t hold a job and doesn’t contribute financially. When we talk this out, I can’t figure out a way out but I feel like I’m drowning. My partner gets very emotional when I bring up my frustrations with this, sharing that he feels I don’t love his son or that I don’t want to be a step mom. My partner doesn’t have the earning potential to get a job that would make enough to pay half of our expenses once you add the cost of daycare so he would still be paying less than half even with going back to work. I have no idea what we should do. How can he assume financial responsibility for his son if he isn’t working to take care of the baby? Is it worth him getting a job just to take SS’s expenses? Do other families do this differently? Am I being insensitive by not wanting to take financial responsibility? Honestly stuck and looking for advice.