My bf wants to know why the ball on his foot hurts to walk and what could it be?

A lot of things: This situation is common and could be due to a variety of things. These include a contusion, which is bruise; stress reaction or even a stress fracture, due to over-activity; metarsalgia, which is pain due to excess weight-bearing under a metatarsal head; a neuroma, which is an inflammed ; tendonitis, etc. If the pain is not getting better he should see an orthopaedist or podiatrist.

Many possibilities: Foot pain could result from different reasons, of which may include: trauma (fracture, tendon or ligamentous tear, arthritis), infection, metabolic conditions (gout) musculoskeletal or biomechanical abnormality, or neurologic conditions (neuropathy). A thorough history and physical exam and possibly laboratory and radiology exams could determine the cause. If symptoms persist get it evaluated.