Not sure what you chose to do, but for future peeps....
The important thing is that you do it, so whenever you think that would be, you should do it. If he has a bad reaction, yes it could ruin this vacation for you and for everybody else involved. Not that it will be your fault, but that could happen. But there is always an event coming up that could be ruined by telling somebody, and that can be the most convenient excuse to keep putting it off until next thing you know, years have gone by.
Another thing, in my experience, people either react well or poorly, but in either case they don't usually want to have long discussions about it afterward. I don't think having a situation that would allow for long discussions day after day should be a factor in when you choose to do it. You need to have time to potentially have a long initial talk, that's about it.
For me, worrying about timing has always been the enemy, and ithe timing of things has never made that big of a difference. You just have to do it, however and whenever you can. I mean be smart about it, if somebody is in the worst mood, or mad at you, etc., just wait for till they have a normal or happy disposition.
