Joseph Hale, 11, joins seven other children in the high street chain's new "Kids Squad", which launched on Friday with the stated aim of celebrating diversity.
Karen Hale said she was "immensely proud" of her son, and that to be able to get the message of "inclusion and diversity out there, is absolutely amazing".
The mother-of-two welcomed the move but said there was "still a lot of negativity out there" towards Down's Syndrome.
"Anybody with special needs and or a disability needs to be included in everything and anything - they can thrive and they just need to be given that chance to show they are just the same as everybody else," the 49-year-old said.
"I think people need to see the positives, it is not the be all and end all, they can go on to have really good enriched lives.
"He doesn't pre-judge, he doesn't see people as different - everybody is the same to him, everybody is his friend.
"If he can see that everybody is the same, why can't people do that with him and other people with disabilities?"
The family, which also includes dad Andrew, 48, and older brother Samuel, 16, live in Cleethorpes, north east Lincolnshire.
Joseph, who loves Jaffa cakes and Batman, attends Cambridge Park Academy in Grimsby, and also suffers with Global Developmental Delay and dyspraxia.
Mrs Hale said Joseph enjoyed the modeling work, adding that her son "took to it like a duck to water".
"He gets quite excited when he sees his photo, he is very happy. His school have been very supportive as well," she said.
Josie Cartridge, customer director of River Island, said they are "thrilled" to have Joseph as part of the campaign.
"The idea of the squad was to create a diverse group of children to portray 'squad goals', Joseph is great fun with an infectious laugh and star presence," she said.
"We hope by being more inclusive, this will help achieve wider acceptance for people with disabilities."