Your son may have had a partial dislocation of a bone in the ankle causing his symptoms. It's not uncommon for a manipulation to realign the joint and restore full motion and function.
A second manipulation may be needed if the injury has been present a month or more. Sometimes the therapist will advise the athlete to tape the joint for the first week back on the field.
Stretching and self-mobilization can be taught to maintain the good results. Many athletes never have a relapse or return of symptoms, even months later.