Grandfather has been getting feeble of late.

Tall : of great or more than average height, especially (with reference to an object) relative to width.
Weak : lacking the power to perform physically demanding tasks; having little physical strength or energy.
Diseased : suffering from disease.
Small : of a size that is less than normal or usual.
Feeble : lacking physical strength, especially as a result of age or illness.

Synonym of Feeble is Weak