Since previous medications haven't helped, your father likely has advanced knee osteoarthritis with reduced joint space. I highly recommend consulting an orthopedic specialist. Next-step treatments may include targeted physical therapy to strengthen surrounding muscles, or intra-articular injections (such as corticosteroids or hyaluronic acid) to reduce inflammation and pain directly. If these conservative measures fail and his mobility is severely affected, a surgical evaluation for a Total Knee Replacement (TKR) is often the most effective long-term solution. Please avoid long-term use of over-the-counter painkillers without medical supervision.