Your husband has two very common problems associated with aging: toenail fungus and numbness in the feet. It is important to realize, however, that these problems are not linked.
Toenail fungus is caused by a variety of common fungi that can also cause athlete's foot. Numbness in the feet has many potential causes including neuropathy causes by diabetes or alcohol use, nerve entrapments, reactions to medications, and neuromas to name a few.
Your husband should be seen by his podiatrist to determine the cause of the numbness and discuss possible treatments for both the numbness and fungus.