He's good police.
This is a variation on the more common:
He's good people.
There is already an English.SE answer that addresses "good people". An excerpt from the answer:
This is an example of synecdoche. Here's some other of the same type from Wikipedia:
A general class name used to denote a specific member of that or an associated class
- "the good book," or "The Book" for the Bible ("Bible" itself comes from the Greek for "book")
- "truck" for any four-wheel drive vehicle (as well as long-haul trailers, etc.)
- "He's good people." (Here, the word "people" is used to denote a specific instance of people, i.e., a person. So the sentence would be interpreted as "He's a good person.")