The boy did not have any health conditions, as far as the family knows, the grief-stricken man told CBC News.
Fredericton police are investigating the sudden death. An autopsy has been ordered.
A witness said the boy fell on a sidewalk on Greenfields Drive as he walked to school at about 7:55 a.m.
Penny Hutley-Dobson saw the fall from her seat on a Fredericton Transit bus.
"I just seen the little boy go right down, just for nothing. I thought he might've just fell," said Hutley-Dobson.
"I just seen the little boy, like you know, how you fall in the wintertime and that was it. It was like a normal fall."
The bus stopped and the driver and another passenger checked on the boy, called an ambulance and tried to revive the child while waiting for paramedics to arrive.
Hutley-Dobson did not get off the bus. She said she didn't see the boy move after he fell.
She said she often saw the boy and his father walking toward nearby Priestman Street Elementary School at that time of day.
An autopsy will be performed, said Fredericton police Insp. Gary Forward, who heads the force's major crime division.
"We don't know what we've got," he said. "We have a young person that collapsed."
The boy was taken to the Dr. Everett Chalmers Hospital by ambulance.
Forward said police were later informed the boy had died.
"We're not quite sure what we're dealing with," said Forward. "Of course, we are going to take all of the investigational avenues that are open to us and look for some answers," he said.
"We really want to make sure we're covering all the bases in this. As you can well imagine, this is certainly a sensitive issue for the family and everybody involved as well as the police officers, so we just want to make sure we're taking all the right steps."
David McTimoney, superintendent of the Anglophone West School District, declined to comment.
"We are unable to make a direct media statement regarding a situation currently under investigation by police," McTimoney said in a statement.
"I can remind you, however, that whenever we become aware of a crisis that impacts students, staff, or the school community, we have protocols in place to support an appropriate response," he said.
"Our number 1 priority will always be the safety, health and well-being of our students and staff."