Update
My son talked to his dad today. I guess AH brought up the fact that he wanted to sleep in one of those tiny compartment hotels in Japan and my ds said, "There's no way I could handle sleeping in something like that!" To which my AH said, "You'll have no choice."
So, he told me that AH was disappointed but agreed without too much conversation about it afterwards. They discussed doing a shorter trip here in the US and started coming up with cities they could visit. Son told AH that he thought that maybe they could put this BIG trip off until after he graduates from high school in 3 years.
I have to say that I'm very proud of my son for saying what he felt, didn't say it mean(no accusations, no pointing out the craziness of the whole thing, etc), and he stuck to his own truth. I hope that at this point, I am doing something right. 2 years ago there was no way I would have handled this so calmly and there's no way my son would have been even remotely OK with talking to his father like this. I'm encouraged by his maturity and by my progress, too. Thank you all, too, for helping to keep me sane!