Hey OP, sorry you've been flamed. This part of the forum is really tough going sometimes and lots of the women that come here are not step mothers and do, unfortunately, leap to some conclusions.
That said, they are all experienced in raising children and tucked away within the nonsense are some astute points.
This little boy is acting out because there has been a lot of upheaval in his life. You and his dad moving in together is a very very big deal for all of you and it does take time to settle down.
It doesn't make you a bad person because you feel like your boyfriend makes some poor decisions and, of course, it also doesn't mean you hate his son. The world is not so black and white.
Similarly, whilst the little one shouldn't be impolite to you, telling you to "shut up" doesn't mean he hates you either.
Your DH and the little boy's mum are his primary parents and they get to decide how to parent him. And that does include what they feed him and how many toys they buy him.
Many millions of step parents go through the stage you are in - I still don't agree with the amount of sugar and screen time my DH allows his children - but you need to learn to step back. It's Step-Back-Parenting!
The exception to this is if you ever think a child is neglected or in danger.
You absolutely should voice your thoughts on how the little boy speaks to you. You can tell the little boy directly but always make sure to tell Dad too. Don't dish out punishments, but talk about feelings. Ask the little boy how he would feel if you told him to shut up. We went to dinner at a friend's last night and I told somebody else's child off for smacking me round the head with a toy drill. You are allowed to set your own boundaries with any other human being, obviously!
There is a role for you in this little boy's life, but you alone do not get to decide what that is. It's up to both of you to do this. Remember he will test boundaries in very clumsy ways because he is young and still learning. In time, and with patience, it will form and if you are careful and kind, it will be one of the most rewarding relationships in your entire life.
I taught both of my step children to swim. I helped my step son ditch his dummy. I convinced my step daughter to try lasagne and now it is her favourite food. But it has taken years and years and years to build a relationship like this.
I can still remember the very very first time I spent the night in their house (whilst they were there) - we'd known each other years at this point. I stayed for dinner and then we told them I might stay the night, if they didn't mind. My step daughter said "okay, daddy" but she looked as though my DH (then boyfriend) had said "we're thinking of murdering the cat!" It's funny when I look back at that point; when I felt like I could never be part of their lives.
Anyway, for you it's been just 1 year - if you're in this for the long haul, there will be many more. And the challenges come thick and fast.
You have to accept that you can't change the way your DH parents this boy. It will make the little boy happier, it will make your DH happier and it will make you happier too. Please trust me on this.
This morning, my step son couldn't find his toothbrush and my DH said "use your sister's". For a moment, I felt like bouncing out of bed and dealing with this atrocity:
"How do you lose a toothbrush??"
"I have some spares in the towel cupboard"
"That's not fair on your sister!"
"Look for it yourself!"
"HOW DO YOU LOSE A FREAKING TOOTHBRUSH???"
But DH had "dealt" with it, in his own disorganised and scruffy way. So, I thought "fuck it" rolled over and carried on with my new book.
.... later I found the toothbrush on the table in the hall. Nobody died and I got my lie in.