Our 16 year old grandson has recently lost bicep muacle mass in right arm. he cannot even throw a beachball.?

Loss of biceps: Loss of, or abnormal placement of the biceps muscle in the upper arm accompanied by weakness strongly suggest a tear, possibly complete, of the biceps. Depending on whether the tear occurred at the upper or lower end, surgery may be warranted (if done within a couple of weeks). Get it checked out.

Sudden loss?: Sudden loss of biceps muscle mass in one arm sounds like a tendon avulsion. The tendon pulls away from the humerus bone. Better take him to his doc. He might need an orthopedic consult. Facioscapularhumeral muscular dystrophy starts more gradually. Also it is inherited as a dominant condition so usually other family members are affected. See doc.