Hey Noddy, i've only just picked this up through a FB link - but whilst I can offer little in the way of advice, I just wanted to let you know you are not on your own! I have a 16 yr old step son, who is quite frankly making my life a misery, he drives me up the wall, I no longer know what to do with him or how to handle him, neither does his father. His father and I have been together 7 years, married for 4, and have 3 children together. SS has ADHD, is on tablets for it (which should he 'forget' to take them he turns from just about manageable to complete and utter nightmare) and over this weekend has done his classic jekyll and hide - he was working with his dad for part of the weekend, and worked well, helpful etc etc - comes to dinner time last night and throws a huff saying he doesn't like sausages (the same ones he ate for dinner 3 days ago) and that he only eats them to keep the peace, and that he'll make himself some toast like the little boys get if they don't eat their dinner - to which I pointed out that on the last two occasions one of the younger ones has done the 'don't like that', they've gone without. This was after I was told that I treat my children differently to him..........I've done more for the ungrateful little monster in the last 7 years than his mother ever did - she's had no contact with him since he was 13 months old.........
Sorry, rant over, there's a whole lot more I could add, but anyway - I've tried all sorts of things from negotiation (which can be very hard when he won't talk/listen/reason with you) to sanctions (no ipod/xbox etc - makes no difference), grounding etc. In some ways I would love to set him up in a flat on his own and see how he fairs, but he is an incredibly immature 16 year old - and although intelligent has made no effort at school so is now heading for a possible F/G in GCSE Maths, not sitting GCSE English, doing Entry Level instead, which compared to his ability is a joke - we've attempted to get it through to him that he won't even get to college on the predicted grades/levels he's been given, never mind a job.
If you come up with something please share - I too am at the end of my tether, as is DH although at times I do feel that he adopts the head in the sand approach, which with a 5 yr old, 4 yr old and 11 week old in the house too, doesn't help much....!!
Good luck, big hug to you and I hope you have a beter day - I know we're heading for a fight this evening from the state he left the place in this morning .... :(