If competitive running gets into his blood, he'll find the best distance, the best training regimen, etc. on his own. Obviously there are a lot more training resources available now than there were back in my day, but any program can turn into drudgery if he doesn't get that "bug" first. He should always listen to his body and be ready to adjust his program accordingly; you can plan workouts flawlessly, but your body may respond differently than you thought it would (sometimes better, sometimes worse). It takes some experience to figure out how to incorporate rest into your regime.
As I first got into shape, I felt better and better, and pushed myself harder and harder, and ran faster and longer. Inevitably I'd overtrain, because that's what I do, and on those occasions I had to back off. I don't know if I would have continued if I had been running a program someone else had given me rather than my own program.
I'd echo the plan to build mileage over the summer. My breakthrough year came my junior season of cross country, after spending the summer exhausted. I worked 8 hours in the fields every day, rode my bike home a few miles, and put in 5 to 10 miles a day, 6 days a week. I ran 500 miles over the summer, every one of them with heavy legs from squatting over tomato plants all day, and none of them particularly fast. I didn't feel ready at all for competitive running, nor did I feel particularly enthused about the upcoming season.
But that mileage base did me a lot of good; once school started and I stopped working in the fields, I could put all my energy into my running and enjoyed the fruits of all my hard work. The competition was a huge motivator as well.
I carried that over to track, then to the next summer, then to 3 seasons of running senior year, and I got better every step of the way. The mileage base was key, and once I had it, maintaining shape was mostly a matter of getting enough rest between races. Eventually I got to the point where my off days were long stretching sessions, followed by a slow 2 miles (sometimes as slow as walking pace).
I wouldn't worry much about the right distance; your son will figure that out as he does group workouts (for example, I was dead last in the 100s the whole tram did, but dominant in the 800s; clearly I was meant for distance). Encourage him to experiment with different distances. If he's one of the better runners on his team, he'll probably end up running multiple distances in the same meet. And there's nothing wrong with running a sub-optimal distance because he likes it more than the optimal one.
.