There’s some good advice on their thread, some of my advice will be repeat of previous advice, but I’m trying to give a total picture.
1) Don’t worry if your son has slow foot speed as a freshman. I was a late bloomer. I ran 5:20 as a freshman and I don’t think I could break 65 in the 400. By my senior year I ran 4:28 and 55 (nothing spectacular, but a Marjorie improvement). In college I was converted to an 800m runner and I could break 50 seconds I’m the 400 as a Junior. I completely transformed as an runner because my training completely changed. I put on 10lbs of muscle and looked like a completely different person. My mile time stayed the same (4:20 at that point in college), but I took close to 10 seconds off my 800 and 5-6 seconds off my HS 400m PR. Point is, a HS freshman Male distance runner probably isn’t very explosive YET and footspeed is easy to develop in the off-season... if that’s something he needs. Don’t fear that’s he’s not fast at 15. He will get faster. Especially if you train him for it. There’s a balance to keeping up his endurance training while developing speed. But it’s not a crazy thought to train him like a 400-800 runner between XC and track, and then pick the endurance training back up once the season starts. He might not set the world on fire in the 3200 that’s season, but long term it might be beneficial. I don’t know your son, so I don’t know if this is what he needs. Which leads to my next point.
2) Try to find a good local coach. Online coaches work also, but you need someone to watch you son run. They need to see his stride, and cadence. They need to see if his shoulders are tight when he runs or if he drops his arms. They need to develop a bond with your son. There are a lot of reasons that an average local coach is better than an online coach, especially at a young age because your son needs to learn the sport. Not just be told what to do, which is what online coaching primarily involves.
3) Switching schools is also something to seriously consider. Good programs have good coaches and good culture. Ask around as to which local HS’s recruit. I don’t know the rules where you live, but most places I’ve lived have allowed some sort of recruiting. If a school provides financial aid to track athletes, then they likely have a coach on staff that can help your son improve. The overall cost of private school might even cost less than hiring a private coach or moving your homestead. A good XC program that experiences consistent success likely has a year round training program. And if it’s a really good school, then coach probably individualizes training for each athlete. This is nothing against public schools (I went to public school). But I’ve noticed a trend of private schools poaching public school runners. So it could be a quick fix.
4) Help your son by helping yourself. It sounds like you understand the sport more than most parents. So you’re already a great tool for your son to use. It might help if you did some research into coaching and training. You likely know the basics, so maybe a hire level understanding would be beneficial. The Daniels Formula is a good starter book for training. The Science of Running by Steve Magness is a great book if you want an in depth understanding. It reads like a textbook though, so be prepare to spend a month reading 250-300 pages. Steve Magness also has a great podcast called “Beyond Coach - Magness and Marcus”. The podcast great for beginner coaches. It doesn’t so much talk about training, but more of communicative and psychological aspects of coaching. It might be worth a listen. You could possibly be you sons greatest asset.
Hope this kinda helps. It would be great for you to keep this thread a live so we can see how your son progresses.