He is someone who could talk a dog off a meat truck.
What does this idiom mean?
It's an idiom for being very persuasive. The idea is that the dog is very interested in being on the meat truck, so talking him into leaving it is difficult.
It's an example of a broad family of idioms along these lines, probably the most famous of which is sell ice to an Eskimo.
Just an addendum to chaos's answer. Searching Google books, the idiom appears to be from 1967 or earlier. In Hanger Stout, awake (1967), Jack Matthews uses the phrase
I don't know why it makes any difference that the day is hot, though.