There is no way it would be fair to his younger brother and sister to cancel the trip. In my opinion he is the one old enough to realize what he is doing and they shouldn't have to bear the burden for his lack of effort. I did consider it though.
Thanks again for letting me know what you would do in this situation. I actually feel like there is no "right answer" for this. He has to learn sometime that hard work gets positive results and lazzines gets negative. Better a lesson through DL than harder lessons down the road.
IMHO, I think that it would not only be "not fair", especially if there's a pretty big age difference between the kids, but also, I think it sends a bad message. I always thought this when I was in school, and that kind of philosophy was used. My thought process on that one was "why should I try and be good and do well if it will make no difference?" someone would always mess it up. As a parent, I can hear "but I was good". I don't think you can send the message "you have to work for it" AND have the philosophy if someone messes up, everyone loses out. Where's the incentive to work for it? Plus, in my house at least, there would be a ton of resentment and bad feelings towards the child that messed it up. As a pre-school teacher for years while I completed my college education, I also think that's alot of power over the family to give to one child. But, that's another discussion entirely.
I know how hard of a decision this has to be for you. I think alot of the responses you've received basically treat a family vacation as an .....well for lack of a better term.....entitlement as a member of the family. I.e. you get to go because you're a member of the family. I personally view anything "extra" that my kids get as a privilege. Most of the time they don't specifically "earn" it (like if they wash the car, they get to go/do xyz), but they have a clear understanding that they will lose privileges if they don't do what they're supposed to do.
Personally, as a teen, if my parents didn't let me go on a trip with the family, whether it be to DL or to the beach, I would have thought "whoa, they're serious here! This is a big deal."
Also, I'm assuming (admittedly, perhaps incorrectly), since you specifically mentioned tracking his grades and talking with him about it, that there was not only bad grades, but some deception or misleading on his part? Maybe not just the poor grades/lack of effort being punished , but the misleading/deception too?
Good luck with whatever you decide! It is sometimes (most of the time!) tough to be a parent!