Unfortunately you have missed the boat a bit here. This happens from time to time when a high school coach runs his 15-18yr old kids like a College or University. The problem with this is that only a tiny percentage of kids really truly care as deeply about their athletic performance as the coach does....The vast majority of kids are there to be involved in a social club so to speak, as they show up day in and day out it is inevitable that some of them will show promise, but that isn't suppose to take the appearance of desire or passion.
My advice to the OP: stop living vicariously through every athlete, expect some to not be invested as much as you clearly are, and focus on the ones that truly do care because, lets face it, those are the only ones who will go on to the next level anyway. Way more important to be a mentor to them, someone they look up to for life lessons as they pertain to running. Help them achieve in the classroom and keep them pointed straight ahead.
Doubt this kid was going to be the difference in a team state title or factor in the scoring enough to affect the other members of the team. I could be wrong here, but even still, his "excuse" is plenty valid in my opinion.