When I moved in with DH, we just talked to DS1 about it, asked him what he thought. We made him part of the decision making process, we explained to him that nothing would change dramatically, it would just mean that there would be 3 of us living in the house, and we would be doing things together. I also made it very clear to him that the two of us would still do stuff together without DH.
It was a pretty smooth transition, there were a few teething problems, but nothing that impacted on my relationship with DS.
I was still the one that did the disciplining, it probably took about a year before a stepped aside and let DH step up.. but that was more from me trying to adjust to having him living with us. It was quite hard for me to let go in that respect.
DH and DS have an interesting relationship now. It's more one of mutual respect and after 12years together, they are definitely more mates than stepfather/stepson.
Good luck hope it all goes well for all of you.
yeah, similar here. dh had known ds since he was about 2.5, been hanging around him regularly for about 6 months before he moved in. i talked about it with ds who was really positive, of course, i never would have let it happen if he wasn't on board. i've always been clear about the fact that ds is treated as if he is dh's, ITMS, there isn't any difference in the way that dh treats him and the other kids, we are a package deal. dh hasn't complained and has stepped up since day dot.
they struggle at times i think... ds is more like me, imaginative and creative, and dh is more mechanical/practical so dh often says he just doesn't 'get' ds. i've come up with things they could do together but dh isn't really the trying kind.. but then he is like that with all of them, i guess? they also struggle because ds can be all over the place and it can be a bit grating at times, esp if you're not used to it IYKWIM. i've found that the best way to throw them together is to do stuff as a family or, with the three of us, we have started playing some board games before bed a few nights a week, ds is really enjoying that - we all are. sounds super dorky but just things that are short and appealing to all of us.
i do the discipline stuff, but again, that happens across the board because i am home most of the time with all of them. dh has started to do it a bit more in the last year though, which i'm pleased about, it shows he's starting to feel a bit more comfortable in their relationship.
before dh moved in we talked heaps, both to each other and ds - AND my ex - about how dh had no intention of being ds's 'dad' (since he already has one of those, no matter how dippy he is), but is 'like' a dad. he's never had any intention of taking over but i DO demand that he be respected, as he bloody well should be, since he does far more to support ds than his actual father does
in time, i hope that they'll become closer and their relationship will be more like mels dh and her ds, but in the meantime i am pretty happy with how things are.