Since last night, my son (15) has been feeling a sharp pain in his chest after drinking. Cold water has been the only thing he's tried. What is this?

See a doctor. Chest pain (CP) is very common in children, and it's uncommon for it to be related to the heart. It can be related to respiratory, musculoskeletal and even GI (Stomach) conditions. Without a complete history and physical exam, it's impossible to give you an accurate diagnosis. Sharp chest pain that hurts more when pushing on the area suggests musculoskeletal, and rest & ibuprofen helps.
Chest pain. On drinking cold water may suggest esophageal spazm or GERD. However evaluation is needed.