She's not going to put him in check. She knows that you don't like her son, which is why she feels like she needs to protective him. She probably thinks he is acting out because you are around, which is her fault. She is torn between her child that she loves and wants to protect, and the man she loves and wants to be with. If you are talking about your feels and she really isn't listening, there is nothing you can do and likely nothing will change. Sounds like you have already disengaged. Disengaging never helps but it does make the situation a bit more tolerable for the person who has disengaged.
I am in a similar situation. I know my son acts out (eats with his mouth open, rude) but mostly because I think he hates my SO because he only acts like that when SO is around. I only stay with my SO because we have a child together. Otherwise, I would never allow someone who didn't like my son to be around him.

I can relate, in a way. I've been dating my SO for about ten months and he has a 9yo son. Although his son does not talk back so much and have all the behavioral issues that your GF's son seems to have, he has a listening problem. And I grew up in a very militant household where you did things the FIRST time you were told, or there were immediate consequences. I never realized how much of my parents' way of bringing me up I had inherited, until I was put in the position where I had to watch my SO's son a lot during the summer.
Let's just say my SO tried his best to be his son's friend to make up for lost time and the divorce. There was never any punishment, never held accountable for actions, etc. I couldn't handle it, and I felt myself building that resentment like it seems you are also. I had to think long and hard about whether this was something I was going to be able to block out, and for me that answer was no. Then I decided it was time to talk with my SO and prepare myself to end the relationship if things didn't change.
I told him that I couldn't stand watching him just exist with his son, and not be a parent.. And that if he wanted our relationship to be successful, things were going to have to change in the house for us and his son. I discussed it in a calm manner and tried not to attack him as much as possible so he would understand that I just wanted these changes to be made to benefit everyone. I think when he sensed that I was seriously ready to walk away from the relationship if things didn't start changing, he agreed and it's been a long road.
Things didn't change overnight, and things still happen that piss me off, but it's all about communication.
IF your girlfriend cannot communicate with you, it's probably time to leave the relationship.

But also take into consideration the seriousness of your relationship. If she's had a lot of guys come and go, she'll probably be reluctant to change things quickly for anyone. But if you guys have a strong relationship, she will probably like the idea of a man helping her with her son, rather than a boyfriend getting pissed off and taking control.

Good luck!

It's been a hard 15 years for us. In our case, things started ok with my wife's kids, but went downhill over time. Now, it is so bad with SD17 that I don't even speak to her anymore. I am counting down the days untils he either fails out of high school or graduates.

Can you clarify what you mean about you being "not allowed" to do or say things about his rudeness? Has your GF specifically forbidden you from commenting about this, or do you mean it causes conflict when you do?

You have been together 4 months, still in the honeymoon phase, still at the stage where you are on your best behaviour and doing you utmost to please your partner and this is what you get. Good Luck.

You have totally disconnected not only from this child but her many areas of your home life, in the short space of 4 months. Your relationship is on a downhill spiral and will end very soon unless you have a deep heart to her calmy and quietly. Write down a list of things you will and will not accept in YOUR home, because it is YOUR home too, then ask her to go through them with you see if the two of you can find some common ground and areas that you can both agree on because if the two of you cannot agree on how this child is parented you are in for many hard years he is only 10 he's not going anywhere soon except into puberty and then the games will begin, if he is this bad now imagine what he will be like when he is 12 or 13.

Nothing here is going to change unless your girlfriend changes it. Unfortunately there is a very old saying which is still true today YOUR OWN CHILDREN MAKE MUSIC OTHER PEOPLE'S MAKE NOISE. What you hear from this child is a whole lot of noise, that is not what she as his mother hears, whatever this child says or does, does not grate on her nerves the way it does on yours - perfectly normal. Now your girlfriend needs to understand that, and if she wants a live in relationship with you or anyone else and she wants it to succeed then she had better learn to respect the other person's feelings. This is no longer just about the needs of her and her son, she needs to respect that you have needs to, and something as simple as sitting at the dinner table with a 10 year old who closes his mouth while chewing, is not a big ask, that should have been sorted the first time you raised it and she should have sorted it, not you. Also, fighting at the dinner table - not on. She and her soon need to keep their arguements away from dinner thanks. However, your biggest problem is that they even argue at all, because why in heaven's name is a grown woman fighting with a 10 year old. Your girlfriend sounds as if she could use some parenting lessons. Still, she is the one who needs to recognise that and until she does, nothing will change sorry.

Her son seems to be ruling the roost here and he is only 10 as I said what will happen in a couple of years.

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Move on dude! This woman and her kid are a mine field you do NOT want to try and navigate!

Why are you living with someone you have only known 4 months?

This is where the old saying "Better to look before you leap" came from!

You leapt into a mess of trouble.

I hate to be the one that says...."don't do it!!" but if you already hate it; it's only going to get worse.

I have been a SP for 14 years and I am just NOW starting to understand it. It's rough; If you don't want to invest time; just walk away. Relationships are hard.

I know what you mean......My line for my SD15 is "I can tell you're lying - your lips are moving!!"

good luck