Or, he will learn that his father stood up for him and his mother when his grandparents and uncle tried to make his father choose. And yes, I speak from experience.
My father's mother was hateful to my mother when my parents were engaged and first married. (She would call my mother when my father was on travel for work and harass her, and she even tried to talk my father into leaving my mother when she was pregnant with me. And that's not the worst of it, believe me.) When I was very young, my parents elected to cut off all contact with his parents. Although his father did not directly participate in the hatefulness, he did nothing to stop it or show any support to my parents, so he was cut off as well. What I learned as a child was that my parents would not let anyone treat their family so horribly and get away with it.
While my parents' situation and the OP's are not at all the same, I do believe that the OP and her husband have some ground to stand on. The BIL tried to change a long-standing tradition by cutting out the OP and her son. Instead of her MIL and FIL staying out of it, they not only chose sides but dictated the terms under which he could be at their house. Who does that??
Yes, the in-laws may be reaching out by sending the cards. Should the OP and her DH continue to cut off his family? Honestly, I don't know what I would do in their situation. I'd be really mad at them, but I think I would try to work it out, if nothing else for my children's sake. But depending on what else may have happened in their past dealings with his parents, I can believe that staying away from them may be best for them. It's too hard to judge this kind of thing without knowing the full history.
OP, best of luck, whatever you decide to do, now and in the future.