My Husband Is 75 And For The Past 3-4 Months

My husband is 75 and for the past 3-4 months and he is developing multiple indentations on his skull. The first and largest is now about 1.75 inches long and .75 inches wide. Most of them are on the frontal part of skull but smaller ones are around the back and sides. He is now having mild headaches in frontal area. Dr did X-ray of skull and no fracture. Any ideas?