A: It's difficult to answer this without knowing and evaluating your son, so I can only speak in generalities. Start by being a detective and think about whether your son's life has changed significantly recently. Was a new sibling born? Did you move to a new house? Has there been any parental conflict? Was there a car accident? Any major change could have triggered separation anxiety. Even a particularly bad dream can linger and cause anxiety.
Watch your son for clues, and work to decrease any source of stress. Meanwhile, have him draw or act out his feelings; children often turn their feelings into art since they have difficulty talking about them. See if you can find any clues there, and certainly give him lots of reassurance. Avoid telling him to "toughen up," as it will only make it harder for him; let him feel your protection and encourage any of his efforts at being brave. If he is anxious for more than a few weeks, it might be time to seek out a child psychologist or other trained and licensed expert who works with young children. You don't want him to suffer unnecessarily -- sometimes an expert can really help.