Need help with how I feel about fiance's son?

They moved on with me about 6 months ago, and I feel like its fallen on me to support him negate his real dad does not. But we eat out often and I'm the one that always pays for his food, I buy his snacks alot, etc. I have my own 2 daughters from my first marriage that I feel like I should be supporting more than him. It makes me feel bad when I'm dropping hundreds of dollars on him a month which is money I could be using in my own daughters.

I do love my fiancé very much and hate to lose her, but idk what to do about her son. He's (9) also getting on my nerves alot, hes constantly running through my house and stomping everywhere he walks. And is never grateful for anything I do.

I'm really struggling so please be nice. I am 40m but do have high functioning autism and this has me overthinking and can't sleep.