Hey Lou, Don't listen to the in-laws. How they can say that this is just "one of those things" is beyond me. I guess that they are trying to be "supportive" of their son. Who knows what he is telling them and saying you should just "get over it", I wonder what they would be saying if things were the other way around. I have the same thing from my W's fathers side of the family. In fact my FIL has pushed my W to get a D. Long story but he left his wife and kids when my W was only 10 years old and was never interested in even seeing his kids. Now he is being "supportive" of his daughter in HIS way, encouraging her to do the same thing he did all those years ago.
As for your S17, my D19 is doing what she needs to to get on with her life. She is working hard and saving for college but my W doesn't like what she is doing, thinks she knows better how D19 should live her life. I think that this is typical of MLCers. Heck, he may be jealous of him, may not want to have the responsibility of being the parent in NZ and would rather he not have to worry about him as he has plans on "living his new life". Remember, for H it's all about HIM and only him. I think that you are doing great by supporting your S's decision on how he wants his life to be. He knows he can count on you if he needs you, of that you can be certain.
