So Sally Vix, you are positing that this young man can make up 45 seconds over 5K in only three weeks time by running the sort of high intensity hill repeats/stadium incline repeats that you suggest? Even though the best documented evidence (a 2017 study of Ethiopian elites and sub-elites performing twice weekly workouts of 40-60 second hill reps) only found an average 2% improvement in 5K performance?
Given months to improve his aerobic capacities along with the fiber recruitment and efficiency gained might allow him to close the gap but, what you suggest has no chance of working in the time frame laid out. Your personal anecdote does not supplant the science.