This is speculation but his symptoms and age appear consistent with metastatic osteosarcoma (bone cancer that has spread, typically to the lungs). Osteosarcoma treatment has not had a breakthrough in decades, so it's a very bad cancer to get. Surgical techniques have gotten more effective at preserving limb function, and there are more tools to ameliorate symptoms from aggressive chemotherapy, but the fundamentals haven't changed since the late 1970s. It's also very sad because this often chemoresistant cancer primarily affects adolescents in their growth spurt years around age 15.

I hope that a more effective treatment can be found soon for this cancer too. It's upsetting that so much progress has been made on so many varieties of cancer, but there are still some varieties that are a big question mark.