I'm not a coach, but I have a suggestion.
Your team probably has slower runners. Pick one who is consistent at pacing and maybe 20 seconds slower than this guy. Let's call this guy Joe. Tell your runner to run behind Joe for the first mile, then say, "Thanks, Joe" and pass him.
If Joe is only 10 seconds slower, then have him follow Joe to the halfway mark. Find that mark on the course and tell him, "Do not pass before here."
Tell the kid that if he doesn't follow your rule, he won't get to race the next week.
His mantra while following the other runner should be something like, "Relax. My race hasn't started yet. Relax. Relax. You are saving your energy, my race hasn't started yet."