Steroid is also pain reduce but physiotherapy treatment  is better option then steroids. So physiotherapy treatment must. Avoid all things.

Its because of osteoarthritis of knee. You should consult a physiotherapist first as Physiotherapy is a conservative treatment without any side effect. It provides better movements , strength, range as well as confidence to the patients. Avoid steroids specially if he is a diabetic and it has side effects. Its relieves the pain temporarily.

Mobilises Patella Isometric Quadriceps Calf Stretches Dynaimic Quadriceps

Physiotherapy would be a safer and better option for him. Steroids will no doubt relieve his pain temporarily.

Dear friend before going for steroids try some physiotherapy sessions for few days....... If it doesn't help than its upto you.

Get the single shot of steroid but do check for diabetic status. Start with knee exercises once pain decreases a bit. I am sending you the link of the same https://play.google.com/store/apps/details?id=com.technoherpes.kneepain

If your father in law does not have diabetes he may have a good effect after the steroid injection if the knee arthritis which I am guessing is early stages and then with physio he can continue his life in a good way however if arthritis is severe then it will not have much effect so it is better to take new x-rays of both knees and consult a local orthopaedic doctor who can help him out One more injection is hyaluronic injections which are like lubricants and they also may help him in reducing the pain