I think that purposely putting yourself in the middle is not helpful to you. They are grown-ups, it's between your son and his Dad. You don't have to forward emails and then get upset that no one acknowledged you and you feel upset. HOW is this helpful for you?...If your son wants to tell his father his travel plans, he is capable of doing so, you don't have to be the self-appointed messenger. His reasons are his business.
My daughter is an adult, she told me to butt out of her business and if I respect her wishes, that's enough . I keep my opinions about her contact with her father's family to myself...(we're divorced, not pleasant and Daddy died unexpectedly but has a large family).
sometimes it's hard to expect that people act the way you want them to...I've learned that silence is golden.