Our ability to stay upright and in the center is called postural control. Postural sway such as you've seen in your grandfather is a natural part of keeping our upright balance. In a normal, healthy adult this movement is so small it's not even noticeable.

Many factors can result in increased postural sway. Change in vision is the most common with older adults. Loss of peripheral vision occurs along with decreased visual acuity. Add any other condition such as glaucoma, cataracts, or macular degeneration and the problem can get worse.

The inner ear plays a role in keeping upright balance. This is called the vestibular system. Humans use visual cues along with vestibular input to help us keep our balance.

One other important factor is ankle strength and sense of position. Any ankle injury or lower extremity weakness can also reduce a person's ability to maintain postural control.

A physical exam is a good place to start for your grandfather. His problem could be something as simple as an ear infection. Or he may need glasses or a change in his current corrective lenses if he already wears glasses. Perhaps a strengthening program for his ankles would help.