It is much more natural from my understanding. And honestly, you don't need massive amounts of protein unless you are trying to build massive amounts of muscle. Your son can gain a lot of strength with a basic diet or minimal supplementing. And if he is serious, then have him explore the benefits of both plant based diet, meat based diet, or combinations between those two options. He should be willing to experiment to find what works best for him.

And for a sport like soccer where endurance, speed, and strength are all important factors you need to have a diet and training combination that accounts for all of them. You can't take a cross-fit approach to a sport. Otherwise he might end up as the strongest but slowest player, or lack endurance to run for 60 minutes.