Here's my coaching advice fwiw:
1) It should be obvious that 8:30 to 8:45 pace is fine for easy running for a 10:40 guy who is better at the mile and who is unlikely to be in peak shape. I would worry more about him running 7:30 on easy days in the spring than I would about him running his current pace on hot, humid days in the summer (presumably not on terrain that's a fast a track). All of the adaptations that you seek are happening at 8:45 pace, so why do you want him to be more tired than he has to be when he's ramping up mileage?
2) He wasn't up for a tempo run at 6:45 pace. I'm guessing that he's capable of it, even in this weather, when he's not in the middle of a sudden 30% ramp in mileage. But a 3-mile tempo is a hard workout for a lot of high school kids. I agree with the poster above who recommended broken tempos at this point . . . or fartlek or something else a little easier. Again, you get most of the same benefits without the needless pressure and stress to the body. But as far as I can tell, you are talking about one single workout. That should never be a cause for serious concern.
3) Asking him to get a blood test or looking into "underlying problems" at this point would be sending a terrible message that his perfectly reasonable pacing is inadequate.