Like most children before hitting puberty, Harry was a typical small town child. The differences between pre-and post junior high Harry include his connection to his father, his view of his father's store, and his overall attitude.
Prior to junior high, Harry
....liked his papa. [He] stopped liking candy and nuts when he was around seven, but, in spite of this, he and Mr. Tillian had remained friends and were still friends the year Harry turned twelve.
This is a very telling statement. That they were "still friends the year Harry turned twelve" tells us that it was then when Harry began to change. We also know that Harry would always stop to see his dad, and he would also bring his friends over. This was much to Mr. Tillian's delight, as he enjoyed having the company of the kids in his store.
What we find next is that, after junior high, Harry would no longer come by the store. As a pre-teenager, he finds it perhaps childish to stop at the store to get candy. He also no longer stops by, and his absence is felt more and more by his father. This is why Mr. Tillian ends up getting the parrot for company.
They went to a burger place. They played video games. They shopped for records. None of them were much interested in candy and nuts anymore. A new group of children came to Mr. Tillian’s shop now. But not Harry Tillian and his friends.
We know that Harry has an epiphany later on when he has to care for his father's store after the latter is sent to the hospital. When Rocky the parrot repeats the words that Mr. Tillman has said over and over, "Where is Harry?", Harry realizes that his father has missed him very much.