How is his gluteal stability on his stance leg and does he get enough hip extension during prior to toe off?
If he is over compensating due to poor extension he may have:
1. Overactive quads and increased lordosis
2. Tight or overactive psoas, therefore ineffective abs generally
3. Reduced or an imbalance in internal femoral rotation
4. Increased lumbar extension and shearing forces on L4-S1 when running, as a result of 1,2 and 3.
5. If his gluteus medius is not stabilizing on landing or just prior to heal strike on the opposite leg, then he may also be collapsing as he lands. This would decrease the effectiveness of his soleus and eccentric ankle dorsiflexion as it tries to absorb some of the weight bearing forces during heal strike (if in fact he has a heal strike).
I could go on but basically you need to watch this person run. The back pain is likely to be a symptom. The cause is likely to be anywhere from the thoracic spine to the toes and will probably be a combination of a few things.
Take a more in depth look, video him and tell us more
thanks for the chance to assist
:smokin