If comfortable, I would first try to meet him and chat casually about his dog, just to get a "read" on him, know what I mean? If you get the feeling he just doesn't give a crap about the state of his poor kiddo, I'd then call animal control. One thing you may want to say is something like "oh, I should mention...I saw your little guy out one day and he almost got hit by a car as he was loose...do you tend to worry if that will happen?"...somehow mention you saw potential danger and gauge the reaction. That poor baby
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