I think you need to decide what you want to do, because, to be blunt, what he says tonight counts for jack shit.
You are the one who either has a termination and lives with that (terminating a wanted baby isn’t easy on your mental health) and it’s not going to do your relationship with DS any good, now or in the future, because you risk resenting him for it.
Or
You are the one who has to be prepared to shoulder 100% of the responsibility for the baby/child.
Because no matter what he says now, there’s nothing to stop him walking away later on. There’s nothing that can make him take on any of the responsibility
As for DS, yes, you and he are a ‘team’ but your ‘job’ is to give him wings to fly, part of that is accepting you both have other relationships that are important to you and people that you both love. A sibling is a gain, not a loss.
You’re excited...surely that tells you a termination is off the cards and that you just need to focus on having a healthy pregnancy and IF DP says he wants to stick around, to start involving him in your day to day life with DS.
Get signed up to the Dr/Midwife and get a scan sorted 😊