Lauren Walker, from Glasgow, was in the Uddingston Foodhall branch with her mum and said her son was getting a bit “restless” during the shop.
“This man took the time with him to talk away [to him] so we could manage to get some shopping done,” Walker wrote on the M&S Facebook page on 18 February.
“When we reached the checkout, my son was thrilled to see his new friend by screaming: ‘Hiya hiya’ to him.
“The staff member spoke away to him and then took him behind the till to let him help.”
Walker uploaded a short, 10-second video of her son being held by the staff member in M&S, watching patiently while the food was being put through.
“[My son] smiled and squealed the full way through it because he was having so much fun,” she continued.
“This man made my shopping experience so much easier and the fact he took time with my son as well was great. Unfortunately I never got his name so hopefully you can pass this onto him and tell him how grateful we were.”
Speaking to The Huffington Post UK, Walker said: “I was really surprised how great he was with my son Alistair.
“When we got to the checkout and Alistair noticed him he just kept shouting and waving. The staff member then put his arms out for him to take Alistair and I couldn’t believe what he was doing.
“Alistair was totally fixated on the scanning and just thought I’d capture it.”
The video has had more than 7,000 likes and 500 shares in the four days since it was posted.
“This is fantastic and it’s certainly brought a smile to our face here in the office,” an M&S spokesperson replied on Facebook.
“It looks like your little one was having a great time and it’s really good to see how our colleague was so helpful and interactive with our customers.
“I’ll make sure this is shared on our internal employee page and that our store manager gets to hear about it too! We can then give him the recognition he deserves.”