This should have been in your OP.
This isn’t a regular teenager feeling left out because mum’s got a new bf.
This is a seriously damaged child who spends an unhealthy amount of time with his mum and you then spend every single weekend away from him.
He’s not acting spoilt - he’s literally being traumatised every time that you are away from him.
You can’t go from him sleeping in your bed, having to sit right next to you and needing you there all of the time - to spending every weekend away.
Every weekend is too much, especially if he is difficult with his siblings - they are not his parents.
You have a child with additional needs and unfortunately that means less freedom for you.
Don’t end things with your bf.
But you carrying on as you are are making things worse.
At the very least I’d do a hobby a couple of times a week outside of the home - so he’s not being replaced with a new man and he’s not worrying about you but you’re away from him.
You need to start going to your room alone and go there earlier to read.
These issues should have been sorted before you started the overnights.
If he wants to go uni, then that’s a great reason to encourage him to be more independent.
How is he with attending school?