A respectable time but I will add my thoughts to your question for what it is worth.
Using my own running 35 years ago I started running in 8th grade and ran 5:15 on cinders, 9th grade 4:49, and by 10th grade 4:28.
Now during that time the coaching that I received was adequate at best. Generally a little bit of distance running and always 400's on the track with varying rest.
Why Coaching Matters
To present day, I have coach a wide range of athletes with varying levels of ability in southern hemisphere.
I have an athlete that at 15 and in 2 years of running (his first 1500m was 5:06) approximately 18-25 miles run has run 4:20 1500 so around 4:37 1600 conversion but also a 16:07 5k on the track.
Another 16 year old boy who during the past 8 months averaged 20 miles a week along with rowing and playing Aussie Rules football has run 1:55 for 800 and 4:02 for 1500 (around 4:18 for 1600m conversion).
As the coach of your young athlete there are a few things to need to know in regards to his progression.
1. Is he more on the speed side or the endurance side?
2. Has he matured early for his age?
3. Does he play any other sports?
4. What are his goals as an athlete?
Stating that an athlete runs 35 miles a week is very meaningless to many coaches. I could tell you that I coached an 18 year old boy to run 3:51 (4:07 1600m) and his weekly mileage was approximately 35 miles a week . What matters are the sessions, the recovery, the environment, and the goals of the athlete.
Good luck and with patience your athlete sounds like he has the ability to run faster!
Which is always the goal.