The problem is not the boy.  The problem is the mother.  The kid is just doing what he is allowed to do.  She is getting something out of acting like this whether it is a misplaced feeling of being a good mother or kicking the can down the road to get peace now and delaying the inevitable major confrontation to the serious detriment of her son,  She may be a wonderful girlfriend but she is not a good mother.  Luckily, people can learn parenting skills.

You can suggest that she read books or take classes.  You can tell her that you need to move out unless she starts treating him in an age appropriate manner and breaks his phone addiction.  Maybe putting him in scouts or martial arts classes or volunteering with a church or community group away from his mother will help with some inner discipline and character building.  But without intervention this will only get worse.  There are countless stories on this board of 10 year olds who became 22 year olds still living in the same bedroom and behaving the same way.

It's probably never going to get better, this is who she is, the kind of parent that she is.

The thing about co-dependency is, it takes two. So, this is not just about the boy, and when he is an adult, not much will have changed because your GF encourages his dependency. It doesn't sound like she disciplines him, either. Or parents him, at all, really. A 10 year old addicted to a cell phone?

My SD28 is a co-dependent, she is VERY dependent on her mom and her mom loves it, she thinks that makes her a great mom.

Thankfully, my SD's co-dependency does not affect me because my husband has never babied her and tried to instill independence in her (it didn't work but she knows he will not enable her).

So, even if your GF's son does ever move out on his own, you can be sure he will still be affecting your life.

It only will get better when she changes and become a parent.  Unfortunately many people do not change and that how life goes on until there 40 

Google  Permissive Parenting

All the symptoms are listed there.  He can't deal with disappoitment because he's never learned to cope with it.  She is setting him up for failure.

And no, he is not happier for it.  Spoiled kids are miserable.

BM parented SS19 just like this, while DH was the so-called "mean" parent, who made him behave, go to bed, do his homework, and act like a human being. He's still 100% dependent on her, and her mini-husband.  She still lets him do whatever he wants, and he is currently living with her, not working or going to school, and she fully supports him financially.

OP, since you are a parent yourself, it's certainly worth trying to get her on the same page as you with parenting consistently.  But she may be unable/unwilling to do it. 

Yeah, probably not going to get better. Like others have said, she’s a permissive parent and has a co-dependent relationship with her kid. Because of this, the child has no coping skills and his behavior will get worse as he progresses in age but not maturity. He will he a hot mess of an adult and you’ll never fully be able to enjoy life with your partner. 

I was in a similar situation and when I spoke up (calmly and maturely) to my partner, he blew a gasket. He couldn’t stand to hear that his kid’s shit didn’t smell like roses. Cut your loses before you find yourself years deep and massively unhappy. 

I agree with the comments above.  It's the mother who is the problem.  She is the one responsible for disciplining her child and she is not doing it.  That is why he knows he can call her and run to her crying because her reaction is always in his favor.  This should bither you for more reasons than one: (1) because it is the direct cause of the kid's behavior; (2) if you intend to have any more children with her, what her parenting skills would be like; and (3) because her kid's behavior is being observed by your own kid.  He did not spoil himself.  If you are willing to work through it, then she needs to change her parenting skills or you might need to step into the discipilinary role (the latter might create even more issues).  Either way, mom is the problem.

In my opinion, the chances of this changing in a positive way are slim to none.

Cut your losses and run.

It's not right for you to have to deal with this, but you are making the choice to. Your daughter on the other hand does not get to make that choice. Don't make her home life bullcrap growing up for a bad parent partner. Surely you can find someone bettee if you don't like being single that won't crap on your child's childhood. 

They just get bigger, uglier and more annoying.

'IMHO of course'. (Rags).

if you stay you are going to have to be the one to parent this kid.  DW is a failed parent.  She just sucks at it.  You are a successful parent. 

So, sit DW down and explain that you are going to start dealing with her whiny crotch dropping and the whiny crap ends immediately. If she doesn't like how you parent and discipline then she can step up and get it done before you have to and since she has a long history of parental failure she will have to do it to your satisfaction.

Then I would sit the kid down and inform him that the next time he cries about a dead cell phone he can watch it get run over in the driveway and he won't get another one other than a basic flip phone that he can only call and text on.

Introduce him to the "Knock it off or get over here and get something to cry about." message. That is a classic proven to work for countless generations.  Dust it off and introduce this cry baby to some historically successful parenting.

If there is no blood or protruding bones or organs there are no tears.  

My SS used to be the crybaby on the block.  He would play with three other boys.  Two were his age and one was the younger brother of one of them.  Finally the young one just threw up his hands and expostulated "Oh my God! Is he crying again!" That was it for me.  SS came home crying about what the young one said. I told him to grow a spine and go play, Then I closed the door and locked it.  SS had a choice. Knock off the baby crap and go play or sit on the door step.

He went to play. He still cried, but he played. And eventually he knocked of the cry baby crap.  We did not rescue him. We let him figure it out. We did not let him hide. He had to either engage with the other boys or sit in clear site of them crying on the doorstep. If he did not like the comments about his behavior, he could change his behavior. And he did. Eventually.

I just joined this site a couple minutes ago and this sounds like what drove me here! 

I am married and have three steps but have issues with all. Only one stays with us regularly. A 12 year old that acts like his 3 year old cousin when he doesn’t get his way or is grumpy. I can’t say I have advice because I came here looking for my own help but I can tell you I’m with you. 

I dread when he comes over.

She is a big part of the problem. She doesnt make him pick up after himself and drops everything to do whatever he wants, gives in. She wont change and he probably won't either. Dont ever move in with them. What you see will be 20x worse. If you cant stand him now, it will only get worse as he grows.