Rip, stick to it. The WORST thing you can do is back down now. I know it isn't what you WANT to do. IF the kid makes a recovery before school is out for the weekend, then reward him with the trip. If he doesn't, he made his decision and he decided what was important to him.
And FiLtH is absolutely right. I did the same thing he did. I aced the tests and stuff, and slacked on the homework. I'm 44 and still have a self discipline problem. And it DOES effect my business, just like FiLtH was saying. If you want him to be self reliant, self sufficient, and self motivated, you're doing what MUST be done. As bad as I was in school, I wish I'd been able to get the chance to do what I wanted in the military, because I could use the life lessons I'd have learned.
I love my son, and I love doing things with him, I wish I could do more with him. But he has the same problems I had, and I REFUSE to let him do what I did. I WILL NOT set him up for the struggles I've had. I have to do what you've done. I'll stop sports, I'll stop games, I'll even stop trips, including racing and hunting, that he and I both dearly love, all just to stop him from going through what I do.
It's a habit you never really break if you carry it through school into adulthood and get away with it. I want him to have more self discipline, I want him to finish college, and I want him to succeed easier than I have. I do NOT want him to be struggling to really succeed at 40 like I have and still am. I could be so much further along than I am, and have so much more. I could do so much more, and give so much more. All for some stupid habits I failed to break.