The man walked home and reported the incident to his father who soon afterwards found the discarded backpack near Marewa Kindergarten in Bedford Rd.
The phone, pie and doughnut had been taken, and cards in a wallet had been searched but were still with the bag.
The man's mother, who did not want her son identified, said her son enjoyed his walks to the shops, where he was well known by the retailers and their staff, and they were important to his freedom and independence.
"He's got a good relationship with them," she said.
It was possible her son had been watched and preyed on, and it was important to make sure it did not happen again, she said.
"He doesn't seem fazed," she said.
"Possibly it's because it happened so quickly it's like, What happened there? He was quite incredulous."
The incident happened about the same time as an accident at the George's Dr intersection.
The man believed it was a boy who had grabbed the backpack, but his mother said "a boy could be anything from 12 to 20 in his eyes".
Anyone with information about the robbery is asked to contact their local police station.