The former X Factor star, 45, had travelled to the capital with her family to lay flowers for the Queen ahead of her state funeral on Monday. Tommy, 12, became overwhelmed and tired, with his dad Craig Pearson then trying to reassure him.
Sam said she had previously believed her son was lazy when he would want to sit down, but insisted she understands him more now.
Taking to Instagram, Sam shared a sweet picture of her son with his dad and wrote: "For those who don’t know, My son Tommy has Autism.
"We were taking the children to London to lay some flowers for the queen and then taking him to the stranger things experience whilst my girls were going to see Matilda the musical. The girls had gone at this point and we had been walking for quite a bit.
"It was busy! People everywhere! I’m always conscious of how busy it is whenever we go to a big city.
"When tommy has to walk or stand for long periods, his legs start to hurt and ache, this tells his brain he needs to sit, this is when the meltdown will start… if he can’t physically sit down!"
She continued: "These are the things I have only just started to learn about my son as his diagnosis was only two years ago. Before this, if we ever went to a big city, we just thought he was being lazy or we wouldn’t take him at all!
"The last two years has been such a learning curve."
Sam said she feels it's important to continue to take her son to big cities because she wants him "to be able to feel more comfortable in busy environments and be able to cope better."
She added: "For anyone out there who has a child with autism and they struggle.. WE UNDERSTAND!
"You are not alone and the more awareness we create… the better the world our special children will live in!!!"
Christine McGuinness, who has three children with TV presenter Paddy McGuinness who have all been diagnosed with autism, praised Sam in the comments.
She wrote: "You’re doing amazing," adding two red heart emojis.
Do you have a story to sell? Get in touch with us at firstname.lastname@example.org or call us direct at 0207 29 33033.