Wow.. I agree with everybody about this...
and I think your son NEEDS to say something
my mom doesnt really like her MIL (my grandmother) either, BUT she is always nice to her, even if my MIL says something a little rude, my mom still smiles and is nice.. later on my mom might get upset and talk to my dad about it, but still.. she doesnt do this in front of her MIL...