Mary - I'm sorry. I've had a similar situation and it is not easy. My dh's ex divorced him, then blamed him for, 12 years after their divorce, marrying a younger woman. She actually told someone that I was the reason for their divorce (which is really interesting as I met dh 10 years after they divorced). I think my official name in her household is The Blonde Whore. *shrugs* I've been called worse by better. My dh's ex is actually so unstable that we now have total custody of the 16 year old (the 22 year old is too old for custody issues).
But it's good that you two are on the same page. He's only 11/12 and right now his mom has a lot of influence on him, especially as he lives with her. Believe me, as he gets older, he'll see through that. Kids are not dumb. As long as your dh and you stand together (and it sounds as if you are), then how you decide to handle this is how you decide to handle this. Remind dss that once he steps over the threshold into your house, dh and your rules are the ones that apply.
And good luck! I'll be thinking about you.