How old is he, Noddy?
My own experience is that for the last 4 or 5 months before my ds's 13th birthday he was an absolute nightmare. Everything I said was twisted out of context and ended up in an argument and it always seemed to be me apologising as I can't be doing with sulking.
Weirdly, my ds did a sort of "reverse Kevin". As soon as he turned 13, he went back to being a rational human being (that was only 3 weeks ago, mind).
My theory is that they'll go through phases like we all do, and no matter how arsy they are being, as long as we keep our cool and keep the lines of communication open, they'll come out of that phase.
Unfortunately, I doubt that my ds is on the straight and narrow permanently, and no doubt he'll have another arsy phase sometime, but for the moment I'm enjoying the peace.
What's the problem with your dp? Do you find your ds winds him up because he's not his natural dad?
If there are still bad feelings over the split between you and his dad (I have no idea how recent that was), you might want to contact Relateen, an offshoot of Relate who are very good with teenagers who've been through a parental split.