What is the significance of the last line in the short story "A Day's Wait" by Ernest Hemmingway?

Let's start in the middle to answer this question about "A
Day's Wait." In the last line the father and narrator says, "The
hold over himself relaxed ...." This refers to an earlier
passage:

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'Your temperature is all right,' I said. It's
nothing to worry about.'
'I don't worry,' he said, 'but I can't keep from
thinking.'
'Don't think,' I said. 'Just take it easy.'
'I'm taking
it easy,' he said and looked straight ahead. He was evidently holding
tight onto himself about
something.

The word "hold"
refers to the emotional restraint the boy was bravely exerting in the face of (as he
believed) advancing death. He was holding in fear, worry, perhaps panic; he was holding
his dignity and self-control together. Therefore in the last line, the hold that relaxes
is the hold of self-restraint in the face of humanity's greatest fear and darkest
voyage.

The line preceding the last line is also important.
It states that the boy's fixed "gaze" at the foot of his bed "slowly relaxed." This
indicates that he was letting go of his courageous determination as the news sank in and
he made the mental adjustment to the idea that death would not be his immediate
fate.

The meaning of the last sentence will now be a easier
to get at:

The
hold over himself relaxed too, finally, and the next day it was very slack and he cried
very easily at little things that were of no
importance.

We already
understand the "hold" that relaxed was his courageous determination to die with grace
and not be hysterical about his fate. The next day, his nerves were spent from the
effort and equally from the relief. It happens very often in life that when courage
calls for self-control in the face of great matters, a happy or beneficial resolution
will release a flood of tears of relief: once the need for great courage and strength is
past, the nerves unravel and the depth of the fear or worry or pain roll to the surface.
This is what the boy--very naturally and authentically--experienced: the relief that
showed the depth of his struggle through unprovoked tears.