What is this weird pain my brother just had?

So my 13-year-old was trying to sleep, and he took a breath, exhaled, and felt as if a spear was going through the left side of his body near his heart (or upper chest-under nipple area). It was such bad pain that he came into my room crying and sweating. I gave him some water, and now he's trying to relax, but he still feels the pain. What is this?