Examening the movement one makes in cross country skiing, which is at least in Norway a part of Telemark skiing my first thought would be SI joint and/or Lower back and anything which stabilises the SI and/or lower back [for sure if one is an inexperienced Telemark skier]. If you examen the SI are you capable to examen hypermobility? I don't think so, only hypo mobility I would say. And changes only if you compare left with right which makes it possible to have 2 hyper mobile joints. Injuries of the hamstrings are not oncommon in that setting: They tend to try to stabilise the pelvis in disfunctions of pelvis movements. You write "no tenderness of the lumbar spine" How about spinal movements from T7 downwards? How about his posture? How about the stability of the back/abdomen? [SLR seems to be possitive] Hyper active muscles of the back? [e.g. Iliopsoas]? Have you performed stress tests, prolonged stress of the SI and lower back in order to reproduce the symptoms?
"He initially strained his hamstrings about 18/12 ago" Which cause? Who diagnosed it as a strain? What did he do to make it better?
I think that the likelyhood of his injury is of neural origin far greater than a pure muscular problem. In case he has a very short relief after your treatment you can almost be sure it is of neural origin.
I must agree that your examination looks sound but have a good look at the back and the SI, this after my personal experience with Telemark skiing and analysing the movements which could a lot of times mobilize the back and when one becomes tired [usualy after 1-2 hours of skiing] It might be a good sport in order to mobilize passively the back.