I suppose smacking him is out if the question?
Do you see him regularly? Are there others around (is he trying to be cool for an audience?) Is he someone you know or someone you see now and then? Age? (Did you tell us that already, if yes, sorry) do you know his family?
Does he appear to have any kind of compassion/empathy?
Are they mean about her or things like "you should have put her down"? (I got this a lot with my one-eyed blind dog -from adults)
Eta I'd make her a cape, Super Polly, because look how amazing she is with only three legs. I'd also employ "broken record technique", come up with one sentence and that is the only way you respond to him. I.e. "polly has been through a lot, and saying mean things really is not nice"
And... if he's very young and being mean it's quite possible that is the kind if thing he hears regularly, emphasizing that it's not okay and that Polly has been through a lot will help him (hopefully)
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