Groin pain is a common problem in Australian football. This game is played outdoors on natural grass. The ball is moved mostly by punt kicking. It's the kicking and rapid change of direction that lead to these types of injuries.
Finding the cause of groin pain can be very difficult. Sometimes imaging studies are able to show stress to the pelvic bones where muscles of the groin attach. Shear forces seem to have the greatest affect on these structures.
Research shows that with chronic groin pain, one of the abdominal muscles gets off in how and when it contracts. This may add to the problem. Physical therapists (called physiotherapists in Australia) are studying groin injuries of this type.
It may be helpful to have your son visit a physiotherapist before making the trip back to the United States Restoring normal muscle contractions of the abdominal and pelvic muscles may be all that's needed.