Pre teen frustrations

I have a 11yr old stepson, Who seems to have hit puberty early, with the whole back chatting, sarcastic remarks and constant sulking, If he is told no or to do a chore he carries on and sulks for hrs on end.

He has been diagnosed with a very very mild form of adhd, and is on medication for it, he is scared of the dark, constantly carries on ppl are out to get him, and so on.

I find because he is so rude to me and his siblings I am snapping at him constantly which ends up in my partner and I fighting. He cries every time he gets into trouble and has attempted to run away this holidays to his nans, which I had to deal with on my own as partner was not home, which lead to me confiscating computer and game console rights for 2 days.

My partner and I are on the verge of splitting because of all the stress this is causing on us, and He has said numerous times he will always pick his son over us. I dont want to split our family up but I doubt things can continue like this.

No matter what we say to stepson he twists it and acts like we hate him and says he never wants to come here, because we dont do fun things with him.

We are a single income family and after having to pay for his flights to visit it has drained us, my partner works nights so we cant do much during the day.

My 2 other children aged 6 and 2 are self sufficient and can entertain themselves without harassing either myself or partner. I find it very frustrating that the 11 yr old wants our constant attention and gets sulky when it isn't all about him.

Guess my question is How Do I deal with all this. I am at my wits end and I dont want my family to be split up over this.