Should a 15 y.O. Be taking a magnesium supplement?

Magnesium. Before you waste your money on a magnesium (M) supplements (S), here is a list of food rich in M: If he consumes none of these foods, you then may need to have some supplements. M supplements can cause nausea, cramps, and diarrhea. It is always better to ingest M from foods rather that from S.
No. If you don't have a known deficiency, adding magnesium or any mineral will not only be unhelpful, but can cause problems. Don't self-diagnose and definitely don;'t self-treat. Get magnesium naturally from pumpkin seeds, spinach, and soybeans to name a few.