If I were in your shoes, I would lean in hard to practicing a lot more before buying a new rifle. I'm guessing, so please let me know if I'm off here, but if most of your practice sessions are paper at 100 yd off a bench the most noticeable benefit of a light recoiling rifle is that it is more comfortable and pleasant to shoot. For a teenager that benefit is unimportant since I am so tough. Immune to pain really. I understand why those soft sissies like their little rifles, but that's not for me.
Start shooting steel at longer distances, and seeing your own impact through the scope, and shooting 50 plus round practice sessions, and doing following practice drill, and I think he will go into the decision better informed.
If I were in your situation, I think my answer to him would be two parts.
Part one, this is a perfect opportunity to learn about epistemology (figuring out how closely our beliefs and our confidence in our beliefs are correlated with the evidence for those beliefs?) what are his goals in terms of what he wants the rifle to be able to do? What specifically does his current rifle not do that he hopes the new one will? Does he have the skills to take advantage of that increased capacity? Is a heavy recoiling, $3+ per shot rifle the best way to build those skills?
Part 2, I'd be inclined to make a deal with him. He needs to shoot 500 rounds out of his 7mm-08 and 100 rounds out of the heaviest recoiling rifle you can get your hands on, this coming spring, from field positions (not off sandbags at the bench). Have him look at his best Form drill score with each and decide if a magnum is actually the tool that will help him achieve his goals. Maybe a new stock and a can for his current rifle (and/or something like a 6.5CM that is cheaper to feed) will satisfy the itch for something new and sexy AND actually help him be a better marksman.
Thread 'Equipment versus practice posts and Rifle practice/shooting'
https://rokslide.com/forums/threads...ice-posts-and-rifle-practice-shooting.165291/