My Father Fell On Thanksgiving. He Fell Backwards Landing On

My father fell on Thanksgiving. He fell backwards landing on his bottom/back area. There is no bruising but he is having a horrible time moving and with pain. He struggles to stand up but once standing it takes him a long time to be able to move his leg forward. He was seen by a doctor who did some xrays and said nothing is broken but his mobility is worse every day and his pain has not subsided. He took aleve about 8 hours following the fall and the doctor prescribed Tylenol 3 - he's been on that for over 24 hours w/o relief. What could be wrong. I feel a fracture may have been missed. He's 98 weighs about 125lbs, is on BP medicine, eye drops for glaucoma.