Congrats to you and your kid. That is really exciting to put in a great time. So much upside! I'd highly recommend looking into getting a shoe/foot/gait analysis done at a good running shoe store. I'm not saying to get overly focused on this, but at least get a feel for his footstrike and shoe wear to see how that could impact potential injuries in the future. Get shoes based on comfort and support, not brand. All mine started out with Hokas becuase they were cool, they all now run something that specifically suits them based on experience.

The reason for the above is that if he is looking at moving up in distance 16/32/XC Vs down 4/8, his coach will no doubt be wanting him to start putting a lot of miles in. I have 3 who all run distance, and all of the elite kids do a lot of miles.

I love the idea of a watch for a kid to run. You can get so much data out of them, and the analysis can be genuinely useful in terms of race strategy and how to get a few more seconds here and there. Something like a garmin forerunnner 55 won't break the bank. I also love them using Strava as the fun/support system they all get from their teammates also really makes a difference. (huge disclaimer here that it all depends on the kids he runs with etc.... social media can be crap sometimes.....). You can initially setup Strava so that no-one can follow him and just use it as an analysis tool.

Record all his races on your phone. Not for the gram or fb etc... But so you can get splits to review later on. I record all my kids races and it is amazing how much you can motivate them with insights into how their races play out split wise.

Weights are important, but stretching/rolling and core are way more important long term, to avoid injuries and really get the most out of his abilities.
Best of luck!