He probably won't listen until he fucks himself up.
I have a few friends his age (who now have the test levels of pre-pubescent girls), who now say, gee I should have listened to you.
I just got another e-mail from a mate who did something like a BBing comp prep just to look good on New Year's Eve going out. I told him that dehydrating was a stupid thing as he would feel like poo and it would be dangerous going out, drinking and taking drugs dehydrated.
Didn't listen, he is still sick now. And moaning.
It is really really stupid to take insulin as it is so easy to manipulate insulin naturally.
Does he also know he is suppressing growth when he is taking insulin?
In all honesty, it is a great thing to watch out for the young and dumb ones, it does get a bit tedious though.
And what do you do when they are all messed up, cause the whole 'I told you so' is a bit cruel then.
Have him google diabetes, and see if he wants to live with that one.