Those who become stepparents really don't know what they're getting into at the outset, as the dynamics of each family can be so different.
Developing a relationship with an NRP usually takes place away from the DC initially. I went a whole year in the relationship my EXDP before I met his DS, and we were gradually introduced. By then I was invested in the relationship, and we were getting ready to move in together. His EXDW had been no trouble up to that point, but when the relationship moved up a gear and we moved in together, she stopped him seeing his DS, as she didn't know me.
To this day, 8 years later, I've split with my DP and I've still never met the EXDW or even spoken to her.
Another thing which causes major issues is the expectations of the NRP on his new partner. I say he intentionally, as 9 times out of 10, it is.
A lot of these NRP's are looking for someone to take on the role of DM while in their care, in the way that DM means the DP get to sit back while she does all the work. I experienced this with my EXDP.
Expectations of what you will and won't do, and a disbelief around that can be so challenging to work through, as some on this post have found.
An example from my experience; my EXDSS wet the bed until he was about 9 almost every time he stayed with us. Initially it was me changing the sheets, washing them, hanging them out and folding them. I read around this issue a bit, and realised there were things we could do to help DSS e.g. not giving him big drinks just before bedtime, and taking him to the loo in the middle of the night, and early morning. My EXDP ignored my suggestions, so he would give his DS huge drinks just before bedtime if he asked "because he didn't want to deprive him". He also refused to take his DS to the loo during the night and in the morning, as "his (EXDP's) sleep would be disturbed".
Me doing the sheets rigmarole carried on until I finally snapped and said he had to do it, as EXDP wasn't taking any steps to try and help the situation. EXDP turned me stopping sorting out the sheets into a "you hate my DS" conversation, instead of considering that I'd actually just got fed up of doing something he should have been doing, or at least helping with, as I'd expect with my own DC.
Over time, I also realised that EXDP didn't like me going out with friends when his DS came because he didn't want to spend time on his own with his own DS. He was happy to spend a few minutes chatting, but just wanted to get back to doing his own thing, while I entertained him. Again when I pulled back from this, it turned into a "you hate my DS" conversation.
These types of conversations on so many different subjects would come up every single time my EXDP's SS would stay (EOW), and it's wearing. All because an NRP's expectations are that the new partner should pick up where RP left off. I could never have foreseen that coming, it's unrealistic to have done.
I will caveat some of that to say my EXDP was carrying a lot of emotional trauma from his childhood with various stepmothers and projecting that onto his DS and my relationship m, whereas the reality was that we actually got on quite well.
From what you are describing @sassbott it sounds like your DP is telling you what you want to hear at the time friction arises, but deep down he still has those same expectations. I'm sure you already know that you need to have a very frank conversation to make an informed decision on the future of your relationship. I hope it goes well.