I think you go see him, tell him you're concerned, and apologize for not getting his message until today. And yes, validate, and even empathize. But I would make sure you tell him that she is STILL his mother, and that his language is unacceptable, and that you understand that he's very angry about the divorce, but he needs to treat his mother with respect, and that if you hear that he's using that kind of language with HER, then he's going to have to deal with YOU.
I think this is an early, yet defining moment in how your post-D co-parenting is going to go down, and a real man-to-little-man moment as well.
