DP's son making me feel like I'm in a relationship with both of them

I disagree. I get that often times this could be the crux but this feels like OP has tries her best to explain the situation and it’s not about that. It feels quite complex. It’s perhaps l, I’m not sure, like ds sees himself and dp on the same level - I don’t mean that they don’t deserve the same respect or time or love etc, I mean that what they would both get from OP even it it was equal would also be different and it sounds like the issue is that it feels like ds wants the same. I can really genuinely see the complex situation but I’m finding it hard to articulate so I can understand why OP is struggling to explain X

And then "12YO, do you want a drink too?"
Is the sensible thing to do.

I suspect you've made a mistake living with them. Because it is "them".

Why would you not offer him breakfast or a drink/biscuit?!

I would also be Frank and straightforward about doing things when you are out. Say you are out with your mum, we are out too, have a great time. Stop all this secrecy and be firm and boundaried and be welcoming and loving the rest of the time. Get Ss support, maybe counselling to have an outlet to talk about the new family dynamic.

A lot of this is tweenager behaviour. However, added to this is the fact you've moved into his territory and changed the dynamic. He is probably terrified of you replacing him in his father's affections and not wanting to go to bed/sleep suggests he may be worried he is missing out. Hiding things from him is going to feed into this fear.

Perhaps you all need to sit down and resassure him that he is very much part of a threesome but that, you and his father also need your own space.

Set boundaries with bedtime etc - he is only 12 - but also make sure he feels included. He needs to see that he has nothing to fear from you. Perhaps ALL 3 OF YOU, need to discuss some house rules. This is a new set up and all of you are finding your level. Just remember that he is just a child and you are the adults.

If you're making drinks/snacks call out to see if he'd like something. Encourage him to occaisionally make you and his Dad a drink.

Perhaps you could occaisionally take him out on your own for a drink/ice cream/burger if you collect him from school or are alone with him.

It's lovely that you've come on here - it shows you care. Hopefully, the advice you've been given will help you understand the situation this young lad finds himself in and enable you to all make your new family unit a success.

I don’t think people are focused on the wrong things at all and you don’t come across well, although I suppose are being honest about your emotions. I don’t think the child sounds insecure I think you do and frankly quite selfish about minor short term inconvenience over his wellbeing now and for the long-term.

I would always check in with a child about things like breakfast or snacks over an adult. Being part of the conversation is normal when you’re in a household. It will look different at different stages and in a few years when he pulls away and it more his friends you (or more likely your husband) will benefit from having a relationship with secure foundation.

This.

The poor boy, being somewhat cuckoo-ed out of his own home. I’m finding this thread quite sad, and I’m sorry, but I agree with others that you need to do a lot better by him. And from what you’ve shared, it seems his Dad needs to as well as he should be picking you up on your behaviour and advocating for his son.

You've got another 8 years or so of this. Your DP has a role to play here in making his son feel loved and included.
Why not talk it over with your DP? You dont say anything about him, when actually he's the lynchpin in making it work. How did he see the dynamics working when you moved in? What does he think about how his son might be feeling? Did he have a discussion with his son before you moved in, or was he just expected to accept the new situation?

Yeah, and asking his dad only about breakfast, it's a bit excluding and like he doesn't matter.

It wouldn't cost OP anything to say "what do you fancy for breakfast, DP? And how about you, Jimbob?"

It's kinda of blanking him in his own home not to, and will make his insecurity and FOMO worse.

Agree wholeheartedly with this ^

I was 30 and living with my mom and helping her pay off the mortgage on her house. Her SO moved in and started making modifications (hanging up paintings, changing the floors) without and just acting like I was an inconvenience to have around. One time I was washing the floors downstairs and he told me I needed to be faster and not listen podcasts while I clean.

I moved out very quickly. Sounds like you may be doing that here without meaning to. He's not paying the bills but should be treated like a full member of the family when it comes to being offered treats, being involved in conversations when he's in the same room etc

You do realise you're the interloper in this scenario, not the child? You're not even his SM but you're othering him and treating him as the outsider.
Your partner also has a lot to answer for. I suggest you both get some parenting classes because you clearly have no idea how detrimental this will be to his DS's mental health going forward.

This

Tea’s fine. I can’t think of a 12-year-old who wouldn’t like a biscuit though. My children would just have taken one at that age. It is telling that your stepson doesn’t know that he can. You need to do more work on how your new little family functions to help him feel comfortable with the dynamics and build a proper relationship with him. He and his father come as a package, so in that sense you ARE in a relationship with both of them.

Sorry but it sounds like you try to excluded him with petty things like not offering a biscuit or breakfast, this is just basic manners. Surprised his dad is not speaking up. Maybe you should move out.

I think the bit about the biscuit is the crux. OP is ignoring the boy's existence and behaving as if he is an intruder in his own home. The bit about not offering him a biscuit sums up her attitude exactly.

This lad has lived with his father his whole life. He is his whole world. You've moved in. You are the newbie, the interloper not him.
Yes of course he shouldnt jump into conversations he isnt part of because that is rude. But if you are making a cup of tea, offering a biscuit to your dp - why on earth would you not also offer one to the boy as well. Hes a child. He lives there. He needs you to mother him a bit not treat him like a spare part that no longer belongs because you've moved in.
Also if you want some alone time with your dp I think you also need to be sure to allow alone time for your dp and his son. His son is bound to be grieving for that 1:1 bond they shared for so many years before and will now feel pushed out. As well as that I would also make a point of trying to find some way to connect with him 1:1 yourself - find a shared interest, a hobby or maybe a school subject you can help him with.

Hopefully you're not one of those people that also thinks school should teach their children more practical "skills " like saving, budgeting,mortgages ,pensions etc.

Just to say that this is my dd. DH and I still together. Only child after years of trying. She doesn’t like to miss out on anything so often if we do stuff we don’t tell her! We come as a 3. It’s a bit ridiculous and very much our own fault as she has been so included in stuff being an only child. Perhaps with your DP and son it’s been a similar set up with him being included a bit too much in things as they were a 2 and now you are in the home too it’s hard to change.

He doesn't like the idea of me and DP going out for a coffee or a meal alone if he's at him mum's or friend's house so we have to hide it from him

🤣🤣

Now you've decided to use another example - yes, you're right, this ^ is not right

However, I expect the poor lad feels so left out when he's not even offered a drink or a biscuit, that he's jealous and paranoid

Poor child.

Sounds like a normal 12 year old to me. the days of children being seen and not heard are thankfully and hopefully long gone. Why would you not include him automatically when you are doing drinks and biscuits and what are you teaching him by leaving him out, that he doesn't matter? As for the conversations have then when he doing other stuff

What is the situation that he’s living full time with your DP? Does he see his mum? Often times in situations like this the parent and child can develop a relationship where they almost feel like equals which could be part of the dynamic that’s playing out. I get what you’re driving at here OP that it almost feels like they both feel like you’re their partner.

You're the interloper I'm afraid. You have time together as a couple, you can't come in after a year and expect the dynamic to change for this boy. YABU.