Well, without an evaluation it would be difficult to determine the cause of his knee pain. Possibilities include nerve impingement/ compression, neuropathy, referred pain from other regions, vascular issues, chondromalacia patellae, loose bodies, infections/ inflammations, arthritis, bursitis, tendonitis, cyst formation, iliotibial band syndrome, injuries to the internal knee ligaments/ menisci, patellar dislocation, bone growths/ tumours etc; just to name a few. I would suggest consulting a primary care physician or an orthopedician for an evaluation. After a cause is diagnosed, it can be managed accordingly.
Hope this helps.