Is his mind open to you and or change?
Showing the kid the reality of improvement should clear it up quick...my guess is he's one of those who "tried it once" and had a negative response..I'm a big believer in "brass tack's". Do you as a coach believe he will have issues with it going to the next level? If yes then the truth is the truth..I'd have a sit down and explain that if he's uncoachable for you what chance of success is there beyond you when his ability to adjust will determine his future opportunities. I think "coachability" rates very high up on the scale for his future. The sooner he understands the better.