beyond recommending to the person to see a physician, everything is speculation.
One possible cause could be an old injury, maybe even unknown the the person which had damaged the cartilage in the knee. Bits of that can get in the way with certain activities, thus causing the pain. But, as said, this is pure speculation. He should see a doctor.
In addition to what Karin has already said, I assume you’ve observed his squat for any weakness in form. Did it start after you began training? What area of his knee? Did he use too much weight in his squats too soon? Is he overweight? You can see there are just too questions for us to give an answer on this one. 🙂 Could be anything, but if it persists I would also have him see a doctor.
Have him track when he feels his pain, and then refer.
Hello Karin, Christine and Harris,
Thank you so much for your answers and your questions. I must clarify that this isn’t really one of my clients. This is my son and, since he lives in another state, I can’t easily assess his form. Yes, he is 30-40 pounds overweight but he hasn’t really added on a lot of exercise recently. The last I heard, the pain is only intermittent now and I will definitely encourage him to see an MD, if the pain persists.
Thank you all again for your answers.