Nicole Duggan, from the US, set up a Facebook page in January 2017 with the aim of making “the judgers” understand more about autism.
Her three-year-old son Riley is non-verbal and she wants other parents and children to know this is not “laziness” on his part.
“When you find out you are going to be a mum, you dream of... their first word, the first time they will clap their hands, the first time they wave goodbye and of course their first steps. All of the ‘normal’ things,” Duggan wrote on Facebook.
“Well in my house these things were far from normal.
“Words were lost, milestones missed and many tears were cried along the way. It is not him being stubborn and it most certainly is not him acting up.”
Duggan continued: “My little boy is just like your child, he loves to dance, he loves to be cuddled, he cries when he falls, and he adores Mickey Mouse.
“He is however ‘wired’ differently. The small things we take for granted every day are the hardest things for him to cope with.
“Different lights, sounds, smells or even the look of something can cause an overload that is too hard for an adult to deal with, let alone my little boy.”
The mum then addressed the letter to people who had judged her son in the past.
“To the people that stare at him because he hums, join in with his little singsong, because in his eyes he is singing the best song in the world,” she wrote.
“To the mothers that pull their children away from him, you are creating the bullies of the future.
“To the lady that called him ‘bold’ in the supermarket, try to look at things from his perspective. An overload of colours and sounds. People whizzing past you.
“And to the friends that have disappeared, I hope this never knocks on your front door. I would not change my small man for the world and if you cannot understand him and how he works, then you do not deserve to be in his life in the first place.”
Duggan said children with autism are the “bravest” type of children who are fighting battles that nobody knows about.
She asked other parents to think before they judged her child again and to try to understand how much of a “superhero” he really is.
The mother has since been sharing photos and anecdotes about her son Riley on the Facebook page My Boy Blue.
Duggan has shared Riley’s interests (”He loves cars, trucks, trains - anything with wheels”) and his personality traits (”He is so loveable, and loves to cuddle”) as well as his favourite foods - cereal, toast and chips.
The mum’s open letter has been shared by thousands of people around the world who are in the same situation.
“I have received so many mails today from mums and dads who have kids on the spectrum,” she wrote on Tuesday 17 January.
“People from Ireland, England, New Zealand, Australia, the US. People are thanking me. This is something I never expected.
“I am only doing what we all do every day, trying to make our kids accepted.”