How to deal with this

I need someone to tell me how can I deal with this and talk to my husband about a thought he has and I find totally wrong. He has a son from a previous relationship who is 9. We have a daughter together who is just shy of 4 months. Well, I can already see so much difference in the way he treats one and the other...for his son the time he gets with him is 100% fully dedicated to him but if I ask him to spend 30 minutes with our daughter he makes it feel like a chore. He spends $$$ on his son and for our daughter second hand stuff will do. He wants his son to have the biggest room in the house (even though he is here a weekend every other) and put our daughter in a box room and he wants this arrangement to last for the next 4-5 years! We go to events or to places only if his son is with us and doesn’t bother to do anything the rest of the time. Pictures are worth keeping only if his son is in them...and the list goes on...today we argued because of something similar (we were on our own) and again he did differences. First he makes me pass for the bitch who gets defensive then he recognizes he does differences and his excuse is: “well she is little, she doesn’t understand /realize these things”. I don’t know how to make him understand that this comment and idea is so wrong! As a parent you cannot think this way!

The truth is that my mil said the same thing...they came to visit us when our daughter was born and they brought a baby grow to her and several presents to my stepson. My mil even said to him “you see we got you so many more presents than your sister because she is little and doesn’t realize that”. When, disappointed, I told my husband he raised his shoulders and said that’s true...

Don’t get me wrong, I love my stepson and I want the best for him too but it aches to see this and I cannot come around the fact they are done because she is too little to notice them 😳