I was to my Internal Med Dr. yesterday. I had pain in my big toe. I was tested for gout and it came back negative. Now the pain has moved to my ankle and elbow.( I should probably tell you I have polymyalgia and fibromyalgia) She told me I problably have .....big long name of a disease, but means "Travelling pain". I know gout, medicines and my other illnesses can all cause this type of pain. These can all be easily tested for and something can be found to be of help. Good luck and hope you find an answer.
Sorry to read your father-in-law is suffering from an undiagnosed ailment. Sometimes when we see the same doctor for problems in ones and twos, they can miss the whole picture that is developing. I suggest you take him to another doctor (perhaps an internist) and present the whole picture. It sounds like some tests may be needed as this is not clearing with steroid injections.