It sounds like your ex has done a number on your DS2, and he has absorbed his father's attitude to you.
This is really unpleasant, but is very hard to unpick, because if you criticise his father to him, then it will just back up his father's view of you.
I think all you can do is to apply consequences for the rudeness, and continue to stay as calm as you possibly can - and maybe ask him in what way he considers you a "loser" for leaving a violent domestic abuse situation?
I mean, it's quite possible that he IS a chip off the old paternal block, and he is just angry that he had to move with you and not stay in the family home with his father - and that he genuinely believes that you should have stayed there just so he could still be living in that house. There are people like that out there - they're so self-absorbed, they don't care about anyone else, so long as they have what they want. I can't tell you if he's like that - but you will know.
I realise that you don't want to lose your boy - of course you don't - but it may come to the point where it would actually be better to let him go back to his Dad's. This would of course be exactly what he wants - but if he is genuinely that selfish, it may be the only way for you to get any peace from his behaviour.
At all times though, maintain the point that you still love him, you just don't love his behaviour right now. I have a 12.5yo boy who is most of the way through puberty, and who already thinks he's an adult, so I have a lot of sympathy with your situation - but mine hasn't reached the stage of calling me an idiot (yet!?), although his tone and his temper leave a lot to be desired at times! WHen it was clear that puberty was starting with him, I showed him the Kevin and Perry tv clips, where Kevin turns 13, which helped because it put some humour into it.
But I think your son is beyond that sort of "help" - he's too far into the moodiness and arrogance of puberty. If you're still seeing the counsellor, I'd suggest asking them for some guidance too - I doubt your son will agree to go himself, and there's no point forcing him to. But you could get some strategies on how to not "bite" when he's goading you.
I'm so sad for you that you're having this trouble with your boy - it's heartbreaking. 