Greetings, sorry about the pain
If you could send the x ray of knee would have predicted when your dad's knee pain will stop albiet replacement is not the solution, but yet another costly approach.
Next Steps
get to see a physiotherapist specialised in trigger point therapy and posture correction will be the best solution for recovery. full recovery will take 3-4 months, but pain reduction of 40% will happen in a weeks time.
Health Tips
try to take cartilage joint supplement to strengthen the knee and optimise your dad's
vitamin d level to see better muscle and bone growth. if any questions do let me know.