“But up until about 3 months ago, we had a great relationship with our son”
Wow while you may know for certain that YOU and your son had a great relationship previously you cannot know for certain that your husband and your son did, for most of my life my mother was in complete denial about the way my father treated me, every day when we were alone he would say terrible, hurtful things to me, and display to me in every way possible that he did not love me.
I am not accusing your husband of anything but it is possible to live in the same household with other people without having a true grasp of what their relationship is like. While there are no good reasons to go off the derech, pretty often there are reasons why dont you ask him?