I've made a few mistakes when my X and I got together, she already had three children and they had a father but listening to the X and being atop a white horse I believed chivalry, ethics and such meant mothers did not lie, especially in front of their children.
I was given the responsibility of a father straight away although my initial way of parenting was that both should be involved.
This was my first and second mistake, I accepted the responsibility and I picked up the slack when she was lapsed. The rules were discussed but she would rather be the kids friend than a parent then rant and rave when things did not go her way. I babble.
The next was I held to the punishment, cause and effect where as she would simply let things go as soon as she calmed down, no further discipline.
I never intensionally set out to be the sole parent in a family but it ended up that way. When my daughter was born a new dynamic entered that I was favoring her, so I'd go out of my way to neutralize the situation and adapt to what my X wanted.
Things got worse and although I gave the kids plenty of love and tried to make everything fair I had my X in the background working against me constantly so she could be the good parent.
Here's the thing.
I was never their parent and I should never of taken this role on, I should have allowed their mother to parent them. Even though my efforts to provide and establish a family atmosphere was true and I did what was expected of me by my wife, it was the wrong thing.
At twelve your son needs a male role model not a dad, if it turns into your son accepting your partner as a father figure then there is much to be proud of.
Unfortunately my experience has cost me dearly and all I did was what was expected of me and I thought was the right thing to do, I am and was a good father and what happened was woe full for both me and the kids.
There are rules for all who live in the house and there are rules for authority you need an understanding of both and to make sure you do not put expectations on your man or your son that neither can live up to.
Best of luck you need to be clear of the rules all round including yourself.
I never offer advice just options that might not have been seen.