Shake hands, get on a first-name basis with it, because it will help you out immeasurably.

Disengagement means essentially "stop parenting him, he has two parents and you are not one of them". If mom doesnt want to parent him to your standards and structure, then back off and let him stay up late, and let her get him up when he whines to sleep in.

If he is telling lies that are beginning to fracture your relationship, then get nanny cams and either just tell him that he is on camera (he will act different), or really record him lying. It might not save your relationship, but it will possibly take a huge load of stress off of you.

But, really back off of the parenting. If he acts bratty let mom and dad deal with it or not. Definitely do not put up with disrespect - you are an adult and this is your household. Kids learn at an early age how to play the adults against each other. My Sds do it, even the nice one. She will receive discipline from DH, and go cry to her mother Toxic Troll BM, and there will be a huge fight with name calling (her), and then anger (him), and stress (me).

Its call "triangulation".

You need to post here (support to help navigate the feelings, ie depression, etc) as well as read read read (for techniques on dealing with GF and her ex and child.)

It is a real concern that you have, of leaving and having another man help raise your child. Also real concern is that she is proving herself a lazy mother, you are seeing how she will be with YOUR child together.

Dont get married whatever you do. If it gets to the point you separate, you will pay child support, and then add alimony,and life REALLY sucks. Trust me, it all sucks, but why add to the Suck-Pile?

You can disengage, but you can not let this kid stay up to midnight because he wants to.   His bed time is 9 pm.  That bed time no exters time after that.  He want to FT. Then that before 9. He has to be done with FT at 9 and to bed.  

If your GF can’t inforce bed time, she is not going to inforce anything else.  

Ugh. Yes, I'm with the others, don't try to be his parent. He has two lousy ones and he's not going to accept you setting rules when they let him run feral.  You don't have to let him be directly disrespectful to you, but don't bother trying to take over and parent.

If he's like this at 5, I can't imagine what he will be like at 15. 

You have a lot on your plate. 

Did you tell the dad his son was using a call as a bedtime stalling tactic? Tell him that you completely respect their relationship but at the same time rules need to be upheld. If bedtime is 8 pm and at 815 he comes out wanting to talk to dad, the call needs to happen in the morning when SS wakes up instead. 

As for your GF, if you decide to stay, which, let's be real, doesn't sound ideal, you need to keep in mind that she is going to be one of the dreaded Golden Uterus mother's. Her child can do no wrong and she is the best mommy dearest in the world. You have 2 options- Counseling to try and make her see the long term damage she is doing by not being an actual parent or disengaging from your SS all together. If you can't parent then you need to be removed from responsibility regarding him. No day care pick ups or babysitting him. No funding his life. No nothing. It sounds cold but you aren't being given other options here. 

I get that you don't want someone else to raise your child, so don't let that happen. 50/50 custody is very common now, fight for it. 

I can relate immensely. I have a 5 year old SS who is very similar. His BM says bc hes only 5 he can't possibly be manipulative, but trust me, he is. I am very blessed that my husband has for the most part supported my feelings and he is very strict when it comes to disciplining SS because he isn't under the impression that SS is an angel. DH loves his son but I dont think he likes him as a person..if that makes sense.  So while he's in our home, he acts right bc he knows we don't budge.

I'm so sorry that you aren't getting support you need and that your girlfriend isn't stepping up. My advice to you is what a lot of the others are saying. Just stop. Stop trying to parent him. Stop putting an effort in when it comes to your relationship with him. Because that's how you get disappointed. Once you take the pressure off yourself, you will feel so much better.

I would say since you aren't married, leave! Run for the hills! Being a SP especially to ill-behaved children is such a challenge and sometimes it feels like it's not worth it. But you do have an 8 month old with her so I know, especially with you being a male, you can't just take your baby and leave like a mother could. So you just have to make the best of it. I promise you, just let your girlfriend know that as of now, you are backing off and she has 100% responsibility for SS. If she ever gets upset with you for not helping, remind her that she never appreciated your help before and that if she wants it, then she can't get mad when you do try to step in.

Remember, you are a parent. Not to your SS but to your 8 month old. You have a say in how that child is raised. And that could very well involve him not being exposed to another child's poor behavior. So If your girlfriend wants them to grow up together, she should really start stepping up and being a better parent to her kid.