He's had to deal with alot at his age, his parents are no longer together and now there's the addition of you and your children to the family. He may feel that he has no control over anything in his life.
My daughter was very stubborn and disobeyed all the time when she was young. Giving her choices worked wonders with her behaviour. This made her feel like she was in control.
Example:
He asks for a drink...you say:
Would you like it in the green cup or the blue one?
Would you like milk or juice?
Would you like it with a lid or a straw?
Would you like to drink it in the kitchen or the dining room?
Each time you ask a question, it involves a choice of 2 items that you are happy with either choice. At first he may not want to choose what you have offered, so choose for him and ask another question (but stick with your choice), he will quickly learn that if he doesn't choose then you will.
This works with discipline too.
Example:
He disobeys or shows unacceptable behaviour...you say:
That behaviour is unacceptable in our family (or at church, school, etc.) would you rather be grounded from the tv for 2 days or from riding your bike (or whatever it is he enjoys doing). If he refuses to choose, simply repeat the question, however, increase the time. If he still refuses do it again...increase the time and repeat the question. He will eventually learn that if he doesn't choose then the time will continue to increase. And this type of discipline is easier to stick too for you because you can point out that it was their choice (both the behaviour and the punishment).
This worked wonders for your daughter who is now 16 and a well behaved young lady. Many times children just need to feel they have control over their circumstances. Once they learn that you will give them choices even in their discipline they start to take responsibility for their behaviour (or at least display negative behaviour less often because they know what the results will be and that there is noone to blame but themselves, even if they don't want to verbally admit it).
Good luck!!