The boy's grandmother, Brenda Millson, received the anonymous letter on Friday and immediately reported it to police.
The letter was signed by someone claiming to be a mother in the area who was upset about noises Millson's grandson Max made when he was outside.
It described Max as "a problem to everyone else" due to "noise polluting whaling" and stated that the noises scared the author's "normal children."
Durham regional police have launched a criminal investigation.
Spokesman David Selby said the letter doesn't meet the "high standards" for what constitutes a hate crime under the Criminal Code.
"Even though it falls below that criteria, it doesn't mean we're not taking this very seriously," he said. "It's obviously a disturbing letter and we have a number of people working on the case."
He added that the Crown Attorney's office advised police on the status of the letter.
Selby said other criminal charges could be relevant but didn't elaborate because the investigation is ongoing.
Steven Penney — a law professor at the University of Alberta — said people often have a broader definition of what constitutes a hate crime than what exists under the Criminal Code.
"We talk about a hate crime in the context of someone communicating something that is perceived to be hateful or hurtful to another person," he said, adding that the Criminal Code talks about the "wilful promotion of hatred towards an identifiable group, or alternatively the public incitement of hatred that could conceivably lead to a breach of the peace."
He said with both of these provisions the communication must be in a public forum or setting, meaning that a private letter wouldn't be covered by the definition.
He added that the term "identifiable group" is closed and doesn't include people with physical or mental disabilities.
"It's limited to any section of the public distinguished by colour, race, religion, ethnic origin or sexual orientation," Penney said.
Millson said when she received the letter, Max had been visiting her in Newcastle from his home in Oshawa, Ont., where he lives with his parents and older brother.
She described her grandson as a wonderful, lovable and sweet boy and said she is grateful her local community has rallied behind the family.
The letter has gone on to make international headlines and the family has received offers of help from across the country, including a charity in Montreal that contacted Millson to say they had received offers from people wanting to help the family pay their bills, buy gift certificates, or send them on an excursion.
"I suggested that Max would just love Canada's Wonderland," said Millson who hadn't yet spoken to the family about the offer. "He'd go every day if you could take him."
Millson received another offer from a woman in British Columbia.
"Gloria... wants to send him a gift," she said. "People have dropped off big balls at the door cause they know he loves balls, and gift cards."
Tiffany MacDonald, who works for Autism Ontario, an organization that provides support to families with autistic children, said the incident should serve as "a teachable moment" because there isn't enough awareness about autism.
MacDonald said the letter is an act of bullying, adding that if the author of the letter was concerned with noise outside, there were better ways the situation could have been handled.
She said "talking respectfully with the family instead of writing a nasty letter" would have been a better approach.
Verbal communication varies among autistic children, she said, adding that Max's noises could have been an expression of a love of the outdoors.
"If this is something that's disturbing, maybe directly connecting with the family, try talking to them," she said.
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