I was going to say the same thing your T said! She doesn't know he is different. You do and his family does. Therefore you want to know what is wrong and he knows that. OW expects nothing from him and askes no questions. He doesn't feel guilty around her for being different. My guess is that he has probably withdrawn from his family as much as he has from you. That is part of PTSD.
I agree with NIkB that it may be a good idea to make some sort of contact. Don't ask him about OW or his trip. Don't expect him to react any certain way. If you leave a message or send an email don't expect him to call you back or respond. Although you calling him would put that seed in his mind. Sound upbeat and pleasant. It is hard to justify calling them when you have no kids. I usually used finances as an excuse. I am not sure if you still have any joint accounts or bills though. Maybe just tell him you wanted to wish him a happy new year is good. Ask him how his classes are going.
