I was wondering - has this got worse lately, or has he always been this difficult? If it has got worse lately, it could be partly due to adolescence, and the changes that brings.
There is a book called Divas and Door Slammers, by Charlie Taylor, which I bought when ds3 was going through a horrendous phase, in his early teens. The author believes that, alongside the hormonal and physical changes of adolescence, there are also changes in the brain - actual, structural changes, caused by the hormonal changes, iirc.
The changes to the brain result in the temporary loss of some abilities - such as empathy, self control etc - hence why teenagers can seem very self-centred, unable to see the effect of their actions on those around them, and not caring even when those effects are pointed out.
He describes it as almost a form of temporary brain damage - but it is temporary, and once the hormonal and other changes settle down, these things return. I definitely saw this in ds3 - he had a hair-trigger temper, could say extremely rude and unkind things, and seemed to have no empathy at all - but we stuck it out, dealt with the bad behaviour each time, and he did come out of it. He's now 19, and a thoughtful, pleasant boy. He still has a bit of a temper, but he can manage it so much better.
Anyway, this is all a rather long-winded way of saying that some of what you are going through might be adolescence, and might get better in time.
I also wonder how your dh deals with his son's behaviour. Does he discipline him when he hurts you or his step siblings? Does he tell his son that his mum is 100% wrong in what she is telling him to do? Hopefully he is backing you up - but your comment about living apart makes me fear that this is not the case.
It certainly sounds as if your dss's mum is a lot of the problem - and that she is unlikely to be of much help in dealing with the situation - so I wonder if you and your dh should look for other support for your dss, with the expectation that his mum will be of no help whatsoever - then if she is helpful, it will be a nice surprise, but if she isn't, it won't impact too much on whatever you and your dh are doing to help your dss - if that makes sense?
I hope you can get help with this - it sounds like a very difficult situation for all concerned - your dss sounds like a very unhappy and confused child, and you are all clearly having a very hard time of it.