Robb Scott, from Canada, whose five-year-old son Turner has Down's syndrome, spoke passionately to the camera about the stereotypes children can face.
He recorded the video in response to overhearing a child asking his dad what Down's syndrome was. The dad responded that it was "an illness of not knowing anything".
"They’re great teachers, people with Down's syndrome, it’s not an illness, it’s not an illness," Scott said to the camera in tears.
"It’s fun, it’s brilliant, it’s amazing, it’s funny, it’s kind, it’s loving, it’s cuddly."
Robb Scott thought he had "failed" his son for not standing up for him in the shop
Describing the conversation he overheard, Scott explained: "There was a father with his two sons and they were looking at movies and the father mentioned he could pick out a movie.
"I heard the son say, about a movie he picked out, 'what's Down's syndrome?' because the movie had a kid in it with Down's syndrome.
"I don't think his dad was trying to be mean, he was searching for the right thing to say and he said it was an illness and that it was an illness of not knowing anything.
"It's one of those moments where you don't know how to act or react, I didn't say anything because I'm not the type to get in people's faces and say things about it."
Scott broke down in tears as he said he believed he "failed" his son in that moment by not challenging the definition.
"Down's syndrome is the best thing that ever happened to me but I didn’t say that, I didn’t step up and that was devastating to me in that moment.
"So I just wanted to right that publicly for myself. I have to reset that button.
"Down's syndrome is literally one of the most beautiful things that’s ever happened in my life.
"Just because you read slower or don’t run as fast does not mean you have a disability to me, this is what I learn from Turner – disability is a perception.
"We’re here to learn things and a well-educated man does not have more to teach than my son, different things but not more; his knowledge is not more valuable."
Robb Scott's two sons
Scott's video, which he filmed in his car straight after hearing the conversation in the shop, has been viewed more than one million times on Facebook in five days since being uploaded on Saturday 20 February 2016.
The dad has received support from people globally, praising him for speaking out about common misconceptions and reassuring him that he had no reason to feel like he had failed his son.
"'People with Down's syndrome are teachers' I love that! I'm going to use this next time someone asks me why my son is going to a mainstream school," wrote one person.
"Thanks for sharing your beautiful thoughts."
"I would like to say thank you for sharing," another commented. "You didn't change those two people's mind but you've changed some people.
"I think you're amazing andn your son's very blessed to have you as a dad."