Paula,
Was your father diagnosed at an ALS center? The reason I ask is that double vision is not a typical symptom of ALS--in fact the eye muscles are usually among the very last to go. However, myasthenia gravis (google it) is a neuromuscular disease that has double vision as one of its major symptoms along with other muscular weakness, and it is treatable. Has your dad's diagnosis been confirmed by a reliable second opinion?