I'm so sorry OP, what a stressful situation for you to deal with just as the birth is coming up!
I sense that you feel sorry for the GF, but also very rightly are concerned for your own needs and your babies needs.
Lots of panicked responses here making assumptions about the GFs prognosis, but I think you really need some more information to figure out what's reasonable. If he has weeks left, then surely your partner should be taking compassionate leave from work now to help, pat leave may be too late? And if he has months left or longer, then the pressure to work on site during pat leave is less, and he could probably use holiday or compassionate leave at another time.
Ultimately I think the best way to deal with the problem is to sit down with your partner and talk, and be vulnerable. Tell him how sorry you are about GF diagnosis, and completely understand that he wants to help. But that he is about to have a baby, and depending on how things go with the birth, you may need more help than he realises - if you end up needing an emcs, and can barely sit up for first few days, he can't really leave you by yourself, has he thought about this?
Often when hearing a terminal diagnosis for a loved one, we just want to fix it and "do something" because it makes us feel better, like we are fixing the problem in some way, so it could be that your partner feels this way, and helping to finish the house is emotionally tied up with not losing his GF, but he is unlikely to have fully processed these feelings, so I would tread carefully in asking him to step away from that task, which is potentially fulfilling that unrealised emotional need. I would lead with explaining your needs and expectations, from a place of love and understanding and compassion.
I think it would be entirely reasonable to ask him to compromise, and split the time between you and the baby, and the building site, depending on what you need after your birth.
Good luck OP