*I also agree with your suggesting that OP is projecting and wanting her new partner to take on the role of father to her DS including at times acting like he is his only child.
I expect however that even if he was prepared to do so, he won't when he feels that OP is clearly resentful of his DD. If the message OP is passing is that his kid is a spoilt ungrateful child and that she only puts up with her because she has no choice, whilst expecting him to want to be a father to her own child, it's no surprise he is reacting back.*
Also again (as always!) this is a massive projection on your part 🤦🏼♀️
Nowhere does the OP say she wants him to act as if he only has one child. She simply wants to carry on with her DS when SD is at mum’s - totally reasonable.
Also there is no indication that her DP is reacting back to anything? Your use of phrase is deliberately loaded and implying he is cross that she wants to do things with her own DS and she has somehow caused this. You do not know that and you’re projecting.
It is equally likely that he had absent parent guilt and doesn’t want his child to miss out on anything and so is preventing any activities when she isn’t there - which is unreasonable and unfair on OP’s DS.
My DP tried this... “we can’t do this because SDs are at Ex’s that weekend”.
“Well that’s a shame but I’m taking my DCs, you’re welcome to come or not it’s up to you”
Sometimes DP comes along, sometimes he doesn’t. Sometimes I don’t even ask him because I want 1:1 time with my kids. However I will never be prevented from doing things with my own DC by DP.
SDs are always welcome if they’re with us, but if they’re not then our lives don’t pause until their next visit.