It is a bit odd that he went to lead him away. If I found a lost child I'd crouch down to their level and say "Where is your mummy or daddy? Are you with a grown up?" and see if they pointed anybody out first. You definitely wouldn't want to lead them away, maybe if they couldn't see them you might walk up and down or to an information desk or something, but definitely not straight away.
I don't know that he necessarily said anything to the child (obviously you can't tell because no sound) but she just looked back to check where the child was. Although her face is blurred you can see that she looks in the child's direction about two seconds before when he was walking the other way to check he was following her into the shop. I often used to walk with DS like this. They're sort of behind you but you're aware of their whereabouts. If the man hadn't intervened the toddler would still have been following her, I expect she looked back because he didn't come up beside her or she didn't hear his footsteps, or she was just generally monitoring where he was. I definitely used to do lots of small glances to check where DS was rather than full on turning around.
It looks like the man looks around first when she shouts so I expect she said something general like "Excuse me, that's my son!" rather than saying her child's name. She might have been calm in order not to scare her child.
Do we know that she reported it? I don't think it says it was her in the article. It might have been someone else or even security staff.
He could have had good intentions but if he did he still acted irresponsibly. It would have been better to stay where the child was and ask him directly "Have you lost your mummy?"