Be realistic, communicate and compromise.
Of course all is well now. When my OH met my kids, it was immediate osmosis! It got much much more difficult when we started to live together. Suddenly, we were faced with different views, expectations and perceptions.
We made it work though by applying the above. My advice would be to honest about each other and discuss ground rules from the start. However disheartening it would have been for both of us if we were not on the same wavelengths from the start, there would have been no way we could have made our evolving relationship successful.
The key aspect for me was that he overall agreed with the way I was raising/disciplining my kids. I was confident in my approach, and although none are perfect and all parents should be adaptable, I wasn't prepared to be much stricter, or much more lenient than I was then, so it was important that he thought my kids were well behaved overall.
One key aspect for him was that he wasn't prepared to be a replacement dad to them. That means that however he accepted that they came as a package, and that meant sharing his life with them, including finances, he wasn't prepared to take on most of the day to day tasks that come with being a dad. That was absolutely fine with me as indeed, I also wasn't looking for a replacement dad for them.
These two were the main consideration, then there was the adjusting to the changes, ie. my kids accepting that they would need to be tidier than they used to be (OH has higher standards then me and gets anxious if not met, although he did gradually lowered his standards with time!), and OH had to accept that kids were kids and that occasionally, they would play up, be noisy and not do as they are told, all part of discipline.
Finally, it came down to being realistic. OH is not very close to DS as they are very different and DS is being a typical teenager, but then we have two male friends who have issues with their own kids the same age, so we are not making it a step issue. There is no conflict or frustration, more an acceptance that they are best to ignore each other for the time being. I am confident that once he grows at of it, and things are thrown in her life, they will become close again but I respect that I can't force it on them, no matter how I wish for it.