What does the last sentence mean

The last sentence in the story is "For the life of him he could not remember."
The word "remember" is referenced three times in the story. Old Woodifield initially has trouble remembering what he has come to tell his boss. It isn't until he gets a sip of the whisky that he remembers. The old man finally tells his boss about his daughters visiting the graves of soldiers in Belgium. Accordingly, the girls had come across the grave of the boss's son while visiting their brother's grave.
Woodifield's words remind the boss of his deep pain. The text tells us that the boss had had only one son and that this son was killed in battle during World War I. Mansfield hints that the boss and Woodifield both rely on whisky as a defense mechanism against their debilitating grief.
Even as Woodifield remembers, the boss tries to forget. So, the last line references the boss's defense against pain—forgetting or not remembering is the only way he can function in his daily life. It is the only way he can reconcile losing a well-loved child.
In the text, we learn that the boss had poured all of his energies into building up a successful business while his son was alive. His aim was to hand over the business to his son when he retired. So, working with this goal in mind lent meaning to his life. Beyond that, life has little meaning. When his son died, the boss was devastated. So, forgetting or not remembering is the only way he can cope with loss and the resultant grief.

By killing and torturing a fly, the boss is hoping to put out of his mind the death of his son in World War I. Although it's been six years since he passed away, the boy's death still haunts his father's mind. On the particular day in which the story is set, the boss' unpleasant memories of his loss are inadvertently triggered by a visit from his friend, Old Woodifield, who makes a clumsy remark about the boy's grave. Try as he might, the boss cannot get the terrible thought of his dead son out of his mind.
That is until an unfortunate fly lands on his desk. The boss finds the torture, and eventual killing of the fly, a useful way to take his mind off from the enormous grief he still feels at the tragic loss of his only child. His sense of helplessness at the death of his son is sublimated into the power of life and death he exerts over the fly.