You're not unreasonable OP.
It's about your DP really, not his son. But I can see how the anger/resentment spills in that direction.
It's very important I think that step parents aren't viewed as lower down the pecking order.
My DH always made an effort to make sure I knew I was just as important (obviously kids are important but within our home, we all mattered just as much). So if I'd bought something he knew I would want as a treat it would be 'no, put that back it is for Cheese'. Or if they wanted to play in our room it would be 'no, you have your own room go and play in there'.
It's got to be give and take. You can't just give give give and give and never take anything for yourself. This is exactly where your resentment is coming from.
It isn't just about sweets. It's about feeling like you can't enjoy anything for yourself and I get how that would in time, build frustration. Who hasn't had a shit day at work and thought, God I just want to go home and have my favourite chocolates in my pajamas. It's not about being petty, everyone should be able to do that in their own home!
As someone said above, it's a real tough gig being a step parent. You don't get the same rewards as a parent but you're expected to always smile and act like a saint. Well in reality it isn't always that simple! Having very real and honest feelings doesn't make you a bad person.
Your DP needs to set some boundaries and stop treating you like a second rate member of your household.
And yes, I've had the bloody toilet issue too. All over the floor and seat etc... I brought it up with DP and it's not happened since because he dealt with it! This is what yours needs to be doing.
What you want does matter. Just as much as anyone else and yes, in this situation just as much as your SS!