It sounds like he might be having back spasms - possibly the arthritis in his back and hips is affecting his posture/biomechanics and causing undue stress on his muscles, triggering the spasms. Poor guy, hopefully there’s something else they can do for him. I’m guessing the Robaxin is a muscle relaxant?
Have you considered adding in acupuncture? It can sometimes be helpful in addition to the medications.