I didn't check the clip to see if it is the episode that I remember to this day from watching it the first time back in the dark ages of no cable but if it is the one I'm thinking of then:

It is a metaphor about the boy slowly going insane.  Not hearing the postman's footsteps until they are closer means that reality is being kept out more and eventually altogether.

Stubborn Mule

thanks for explain it to me guys.it was a good eppy, I think overall I prefer twilight zone over night gallery,.

I found that most of the other episodes of Night Gallery were actually boring.   
I absolutely agree, especially after going through 4 DVDs of the second season.  There were maybe 4 stories in all that were interesting, and not predictable.

One thing that I always remember when I look back at old series and that is to remember what was running against it at the time.

I watched Night Gallery religiously because it was more interesting than the other stuff.  It was like watching Space 1999 because it was the only science fiction program on at the time.  It sucks as a program but it was better than what ran against it at the time and for my age.  There are not many stories of Night Gallery that I remember but this one was one of them.  I think it might have been because I had read the story before seeing it but the years have made that memory vague.

Stubborn Mule