horse shoe kidneys depending on the severity may affect his career due to the abnormal location of the kidneys. the usual location of the kidneys for patients with horse shoe kidneys are at the anterior abdomen or behind the stomach. the normal location of the kidneys are at the flank area protected by the ribs and a great amount of fat to cushion the kidneys.
that being said the abnormal location of the kidney may limit your son to be involved in professional boxing or even amateur boxing because it will place his kidneys at higher risk for injury
surgery can be performed to place both kidneys at the normal anatomic location although this is a major operation and entails alot of risk also.
i hope i have answered your question and best wishes.