Why does it upset me when my fiance does something unplanned with his son?

I see that there are some concerns with personal values as well as views on parenting. This can be common to see in blended families, meaning that they come from different backgrounds, upbringings, religious beliefs, or economic statuses. I would start by asking yourself some questions to see where the frustrations are coming from. Is there a value in cleanliness/chores, routines, or how about parent/child interactions? Perhaps there is a concern with the amount of time you see your partner spending with his child. There are some personal values or traditions in which you both may have experienced differently. Do you feel left out when they spend time together without you? Or is it excessive, where you are not feeling prioritized? In either of the two situations, you would benefit from openly expressing your concerns. Perhaps your partner may not fully understand where your concerns are coming from.

Now to address some questions on parenting. Without knowing how much involvement you have with parenting your fiancé’s son, I can only assume that you are not part of the discussion about parenting. How involved do you want to be? Can you express what you think your role should be? Has your partner expressed how involved they would like for you to be? Perhaps this on its own can help not make you feel left out or frustrated when they do something last minute. I would encourage you to have a conversation with your partner about your role in raising his son. You are now a parental figure in his life and will be involved with him. This can also help bring you all together and learn how to express concerns openly. This conversation can center around caregiving tasks, setting up and enforcing household rules, and even bigger parenting decisions like how to handle challenging behavior. There may be some relationship dynamics that you both may not be aware of but can only provide assumptions until you both sit down and get on the same page.  

Some goals for this conversation with your partner are meant to set up boundaries and expectations. In relationships at times, we forget that our partners aren’t able to read our minds or truly know what we are thinking or feeling unless we express them. If you feel that this is too uncomfortable or there is no clear resolution after an open conversation, going to couples counseling would be the next step in order to help resolve this.