Caroline White, 41, said the shop assistant's simple gesture has filled her with "faith in society" by proving that attitudes to Down’s syndrome are changing for the better.
She was in the brand's Towbridge Store, when her seven-year-old son Seb decided to stage a "protest" and refused to follow her down the stairs.
That was when shop assistant Ashley Rogers approached Seb, engaged him in a conversation and persuaded him to go after his mum.
"It completely changed the direction of my day and stayed with me," White told HuffPost UK Parents.
"My next move would have been to carry Seb out, which I didn't really want to happen.
"What touched me was how Ashley sat down next to Seb, so that he was on the same level."
Caroline White and her son Seb
"I have no doubt Ashely would have reacted like this with any child and I am certain he didn’t even know Seb had Down’s syndrome until after he had sat down next to him," added the mum-of-three, from Bath, Somerset.
"I don’t think it was special treatment, he just saw a 'lost' child."
When White returned home after her shopping trip she felt compelled to share her positive experience on TK Maxx's Facebook page.
"I cannot express how much this young man's actions meant to me," she wrote on Facebook under her maiden name Playle.
"To see him speak to Seb so kindly and concerned, getting down to his level by sitting down next to him and then motivating him to move off the stairs at a time when I was starting to feel the stress rise up inside me.
"It was done with genuine concern too as he was unaware at first that I was watching."
"One of the reasons I posted so publicly is because all too often we read negative stories of discrimination on social media – and I am the first person to get ranty and up in arms about it," said White.
"The reality though, is that, thankfully, negative experiences are very rare. People engage beautifully with Seb. He is charming and he does attract attention, but people respond with kindness - to the point that my other son asks me why does everyone always talk to Seb, and not him.
"I want new parents facing a diagnosis, and the wider public, to know that life is good, life is ok and the majority of people out there are good, decent human beings."
The post quickly received more than 30,000 likes and 3,000 shares and White says she has been touched by the positive response.
"I once feared Seb’s future, now I am excited by it and the comments that have been made in response to the TK Maxx have filled me with so much faith in society, the effect of inclusion and changing attitudes surrounding Down’s syndrome," she said.
"For this to happen just before the launch of Mencap’s Learning Disability Week and the week the press is reporting yet again about new less invasive testing for Down’s syndrome during pregnancy and using terminology like 'risk', is just perfect.
“I like to believe that Ashley, will have grown up in a far more inclusive society than I did. When I was at school, people who were 'different' went to their own school - almost like a sub group of society.
"I truly believe that Ashley didn’t see any 'difference', he just saw a 'lost' boy. His reaction was tolerant and understanding."
“It’s important to note that shopping with Seb is very much like shopping with any child, he gets easily bored and distracted and doesn’t enjoy the whole experience," said White.
"Having Down's syndrome does mean he has little sense of danger and will often run off, without thinking through the consequences.
"In shops where there are rails and rails of clothing it can be stressful as I cannot take my eyes off him for one second. He sees it as a game to run off.
"Also, he would take longer to realise he was lost than my other son, who is younger, who would look lost almost straight away. Seb would see it as an adventure for a little longer - he is thankfully getting more aware though.”
As well as being so widely shared the message was also brought to the attention of Ashley Rogers, who replied:
"Many thanks for your kind words and indeed thanks to those who have left incredibly kind comments as well - it's been massively overwhelming to see so many likes, shares and comments on this post and to think that thousands of people have been touched by this story," he wrote.
"I honestly had no idea my, what I consider to be, small actions made such a hugely positive impact on both you and your son, Seb. "
Caroline has worked with the learning disability charity Mencap to help raise awareness of what life is like for families of people with a learning disability for a number of years.
She will be supporting Learning Disability Week 2015 (15 to 21 June) to help Mencap give the public a greater understanding of learning disability, and to help raise awareness of the inspiring lives of many of the 1.4 million people in the UK who have a learning disability.