Golly rama, how difficult for you all to navigate this territory. I can only theorise on how I would try to help, if in this situation. It sounds like emotional abuse and if it is, then 'loyalty' or shame might be keeping your son quiet. Sometimes people find themselves bullied into silence just to 'keep the peace'.
What came to mind first is opening lines of communication with your son that show you care for him AND his wife, - that you value her as his partner and want to support them both. In that framework, I think I might then say something like ''Please do not think I am being critical, but I have noticed that sometimes ....... speaks as if she is upset with you. Is SHE okay? Are you okay?''
Seems to me that he needs to hear somehow that this behaviour is not reasonable and doesn't need to be accepted.