It's been a year since I started dating new guy. I've been really, really careful with S12. I didn't hide new guy, but I did create a big moat around S12 so that access has been limited. I wanted to wait and see how things were, and I think it's ok to loosen up a little.
By loosen up, I mean doing stuff with new guy and S12. Things like: tomorrow night, I'm taking S12 to a family-friendly comedy improv night with his friend. The boys usually sit in the first few rows, whereas I like to sit in the back with all the other introverts. I can see inviting new guy to come with me while the boys enjoy themselves.
I talked to S12 about this. A while ago, S12 blurted out that he didn't like the idea of new guy, and after some sleuthing, he said it was because he didn't like the idea of me dating. We worked through that ok, and can now joke about it. A week ago or so, he had a bump in his ear and I said, "New guy could look at it." New guy has medical training. S12 said, "NO WAY. I'd rather ask dad." My kid likes to make psychological analysis very easy for me.
I think I'm doing the right thing here by pushing forward with normalcy despite S12's resistance. I have said to him, "It's awkward, but it's normal awkward. And you don't need to like him, but I do want you to show respect, like saying hello and thank you." Which he does.
Anything else I should be thinking about? New guy has three kids, 19, 16, and 14. He's probably not someone that S12 is ever going to really bond with. S12 is a geek, has a very funny quirky sense of humor, and new guy is just... .well, not geeky.
I've made a point of spending time with S12, and he knows he's special to me. We spent my birthday having dinner together instead of me going out with new guy. Something S12 pointed out.
Any pointers on this from those of you who have done it would be great. I think it's partly S12's age -- he isn't a little kid who will just go with the flow. He has some opinions about stuff, so I'm trying to be sensitive to that.
Also, I have a suspicion that N/BPDx has had overt conversations about the loyalty bind stuff. He did that to my former step son, saying stuff like, "I need you to have my back. The family turned on me and they're all evil, blah blah blah." And my former step son did exactly that. Fortunately, I won him over with my sheer charm

but also, N/BPDx kinda idealized me back then and former step son was all for it.