If I understand you right he went into the mountains walking with Nordic walking poles. Did he also wore a heavy back pack?
Has the focus been on the prothesis only? The reason I ask because over use of the Lattisimus Dorsi (potential shortening) could reduce the ROM in abduction and flexion of the shoulder. Have you looked at the performance of the whole shoulder girdle? I assume some form of imaging has been done f the Gleno-Humeral joint and no specific faults were found.
Just curious...