IMHO, I do think it’s important to consider the hierarchy of needs and remember that you can’t as easily accelerate in STEM in school.
So the magnet school, taking advantage of their plentiful STEM courses, is the priority. Colleges will recognize these courses, along with whatever foreign language and art they offer. And if there isn’t enough art or Mandarin at the magnet school, then you son is just the person to start the Mandarin and Art clubs at school. It will show initiative, which adcoms recognize, while contributing to the community (if there’s enough interest).
From our family’s experience, keeping instruction to a minimum of driving destinations was something we had to make happen as my kid’s sports training accelerated. Trust me, if you can make things happen within the STEM magnet, take advantage! From what I understand, soccer is a heavy driving sport. You want to save the family energy for sports travel. Some years can just about break you (ask me how I know!).
Early or advanced instruction in STEM and sports is the priority, because you can’t get this time back in these two areas. As the parent of a high level athlete looking to study MechEng, we doubled down in math and science classes some years, and are having a lighter science year this year due to sports demands. The transcript won’t be perfect for schools, but balancing both ain’t easy.
And I must say this, although it will be controversial - some posters here will insist that “it’s too early to know what they want to study in college.” That may be so in their kids’ case, and in the case of most students. No problem. However, there are exceptions. I was one. My kid is trending to be another - in that, MechEng suits their approach to math, science, art, etc. While also reflecting their lesser interest in other sub-genres of STEM.
The thing is, you can always pull back from certain paths. However, most STEM schools expect a student who does do a certain amount of STEM subjects and personal interests. And that usually means early exposure - classes, clubs, extra-curriculars.
You will probably have to compromise on your kid’s Mandarin, however. If he really cares about it, he will continue to self-study - watching language instruction videos online, as well as perhaps some shows. To not do the STEM magnate because he’s passionate about some language in middle school is quite the gamble. Again, he can continue on his own slowly over time and test out AP Mandarin in grade 10 - covering both his Mandarin passion and the rigor wanted from many adcoms.