Soft tissue damage or a meniscal tear wouldn't show up on an x-ray, but can be debilitating and painful; or sometimes something as simple as a muscle imbalance in the thigh can cause the patellar (knee cap) to track incorrectly, resulting in pain. Is he experiencing "catching" or "locking" of the knee joint, or more of a constant, throbbing ache? Did something trigger the onset of pain?
Perhaps one visit to a PT- who can do a thorough evaluation, followed by setting up a comprehensive home program- might help. I am not sure where you are located, but you can check the APTA website, www.APTA.org to locate a therapist near you. An evaluation might cost $100-$150, or maybe less if you explain that you are self-pay and only need 1 visit. But, they can help determine if this is something serious that needs a follow-up with a doc, or something that can be managed by the patient.
In Illinois at this time, a patient can go to a PT to be evaluated without a prescription from a doc, but on-going treatment sessions do require a prescription from a doctor.
In the meantime, he can try heat/ice/heat/ice after work while he rests it; gently stretch the muscles of the hip, low back, thigh and calf; wear supportive footwear; avoid twisting motions; massage the area (or have you do it); and try to avoid over-doing anything like running, biking, hiking, etc.