For those of you who gave thoughtful responses to my earlier post, here's what happened. I spent the best part of a week composing an email to my son about his tendency to belittle his wife when they disagree (in front of us or others). It hurt me that he would treat his wife this way, and I wanted to let him know that the impact was probably greater than he realised.
After endless drafts, I sent it this morning. The phone rang shortly thereafter, and we had a long chat. He wasn't offended that I was "interfering" and he didn't make excuses. He said he absolutely took on board all I was saying and knew his behaviour was unacceptable. He then explained something about the tensions in the marriage, not using this as an excuse but saying that they had got into a pattern of "you hurt me, so now I'll hurt you".
His wife is away with her mother and sister this weekend, so I knew it was a good time to raise the subject - and because she'll be coming home Sunday, feeling all happy and relaxed (I hope!), it will be a good time for them to sit down and talk about some of the issues that are causing problems.
I don't know how it will go from here, but I'm glad I said something, relieved that he responded in such a positive way and now just hope that a glass of wine and some candid conversation will put things back on track!