I've mentioned this in a few other threads. There are diabetic therapists out there. My sister went through the same thing when she was a junior/senior in high school. At that point she'd had D for 12+ years and was sick of it...sick of our parents nagging her about testing and her numbers/A1c/bolusing. Her A1c was 14.5 or something higher than that at its worst point.
Before she went to college, she went and saw (not-voluntarily) a psychotherapist who specialized in diabetics.
Might I also suggest that the depression and diabetes may also be completely unrelated and that the depression is causing the apathy towards him not wanting to take care of himself. I've been diagnosed with depression (mid 1990s) and I had that much longer than I've had the D (April 2005).
Interrelated or not, his health should be the top priority here, so if it was me I'd probably look into some type of counseling.
Have you suggested to him about possibly joining Juvenation? There are a ton of people of all ages on here who feel or felt the way he does and can understand etc.
Anyway sorry for jumping all over the place there...hope some of that helps