My son is supposed to be with his father on fridays but my son has a birthday party to attend near me. I'm just going to tell his father that plans have changed and he needs to be with me for another day regardless of what he has to say.
Why are you being so forceful with your decision.
These were plans that had been set in stone weeks ago and I just forgot to tell him about it. I'm not going to change them now just because of that.
I understand that the plans were set in stone earlier but you really need to make sure that you communicate with the kid's father. You agreed to the terms of custody and just asserting your dominance to get an extra day is completely unfair to his father who is still trying to be there for his child. You should really consider his feeling before you just decide things for him.
You are just going to make me look bad by calling me out on this aren't you
I a not taking a side.   You asked for advice and I told you what could go wrong.   You should apologize and ask how to work this out.