The right thing to say

We were doing some shopping in Sainsburys this afternoon, and were just at the till packing up, when Esmeralda noticed an older gentleman tutting behind her. She apologised and moved to the side as she thought maybe she was in his way.
"Oh it's not you that's bothering me, it's whoever that belongs to," he said, gesturing to the side and sneering.
"That" was Peter, sitting beautifully, strapped into one of the trollies made for older children, staring into space and tapping his foot gently on a sign.
"Oh, he's mine," said Esmeralda, completely shocked.
"Oh it's a boy is it?" the man replied.
The man then leaned down to Peter and stared into his face. "Smile!" the man said. Peter ignored him.
"It's not even going to smile then," the man said to me, sneered again and walked off. I wanted to run after him and punch him or have him arrested for just being a horrible person, but in the moment I had no words.

It was one of those situations where you can think of a hundred things to say after the event that would have been appropriately cutting and witty and showed the man up for who he was... but the moment passed, the man walked off and we'd said nothing.

I still don't know what the right thing to say would have been - something to set a good example, that would have shown Peter that it's unacceptable to treat people like that, and that we will always be there to stand up for him. Hopefully he doesn't remember it, and hopefully these moments and ignorant people will be few and far between.