I am engaged to a guy, and we have a 5 month old son together.
He has a 4 year old son from a previous relationship, and we get him on weekends.
(Friday night, take him back Monday night)

When we get him, the entire weekend is spent taking "us" to do "fun stuff."
(Kid stuff)
He doesn't ask my opinion about what we can do on the weekend, and I don't feel he takes me into any sort of consideration. It is only and always about what his son wants, or wants to do.
He gives him $5.00 allowance every week for "cleaning his room" (which I sometimes end up having to do), and I (finally) convinced him that it would be a good idea to try to teach his son how to save money, so that he can put it towards a larger goal (instead of silly toys he breaks within an hour). If we go to the store and his son sees a toy he wants that is more expensive, he will tell him "You don't have enough money for that." But he buys it for him anyways. When I say something about it to him, he says "I felt bad. The look on his face!" This makes me want to throttle him.

Anyhow, when we go do all this "fun stuff", my role is pretty much to push the stroller around and hold all their stuff so they can do "fun stuff." Great, lovely and fine. But why is it that when I say I am thirsty, and ask him to buy me a beverage, he says "You REALLY want that? I would much rather get THIS." Ok but wait.. I said *I* was thirsty. I didn't know I had to drink the same thing you want to drink. Wait... Why do I have to do that? Huh?

He basically makes me feel like an idiot who is in the wrong for being thirsty.
So his son can buy expensive ass toys and get milkshakes and ice cream and candy and whatever the hell else he wants, but I cannot get a coke for a dollar.

Side note: We decided as a unit that it would be better all around if I became a SAHM once I gave birth to our son. I don't have any income, or sway over how any income is spent (save for groceries, which he has to approve.)

So more to the story (I know this is all really random, but these are just a myriad of things that either piss me off or concern me.) My fiance will say some pretty crappy things in regards to his son's BM (such as "I don't care what your mother said, she is wrong." "This isn't your mom's house." "I know your mother thinks it's ok to be a moron, but that doesn't mean I am going to let you grow up to me illiterate and dumb.") For some reason, I find it a lot easier to stand up to him on THAT than I do my own problems with him. I can EASILY tell him (in private, of course) that he is being a dickhead and doing more damage than good by saying those things.. And that while neither of us like the BM, he should really shut the hell up about anything negative he has to say about her until no child is around to hear it. He doesn't need to hear that kind of crap.

Oh! And during the week, when it is just him, myself and our 5 month old son, he will NOT help with our son. He will hug him and hold him for (literally) one minute. As soon as our son makes a SOUND, he says "Ok I can't deal with him." and hands him back. If our baby cries, he says "Shut the hell up!" and gets all pissy about him crying. But when his first son cries, he is instantly right there hugging him, holding him and looking for a boo-boo.

These random things are making me resent (and honestly, despise) his four year old.

I don't WANT to resent the kid. He really is a good kid.

I need some pointers and/or advice with this. What can I do???

I appreciate the help!
WileyMama