If its tha later, make sure he never gets on a med that has negative sexual side effects
, if its the first its easily curable, if its stress then he needs to find the source of the problem and fix it, and increase exercise.
The problem is, often guys dont want to get help. When we loose our sexual nature, its hard to admit and it often is some relief in comparison to our normal thinking about sex ever 5 min mode. However, this can be hard on a relationship and cause stress in a marriage.