You prescribe NSAIDs and/or acetaminophen to manage his pain

You explain that his symptoms sound like knee OA, even though he is 58 years old, which is a bit early onset for this kind of degenerative arthritis. 

You suggest that he should take it easy with lifting weights and overexerting his knees. For pain management, the first pharmacological step is acetaminophen and/or Non-Steroidal Anti-Inflammatory Drugs (such as Naproxen, Ibuprofen etc.). You explain how he should use these two type of medication and make sure he understands that he should avoid overusing them. 

You also suggest that mild exercise like walking, swimming or Yoga, Pilates would be a good idea for maintaing a good physical condition which always helps with pain management of OA.