Please don't rely on animal control to settle this unless you are willing to live with that dog being put down, if they pick it up for being loose -- if your neighbor can't be bothered to pick it up from ACO, there is a strong possibility the dog will be euthanized. DO NOT ASSUME IT WILL BE ADOPTED BY SOMEONE ELSE.
The assumption people often make about there always being some loving, wonderful family waiting to adopt a purebred dog from a shelter is flat out wrong, in most parts of the country. Purebred dogs, including friendly, healthy GSDs are often euthanized to make space. Sending him off with animal control should be a last resort, not a first phone call -- please try to settle it with your neighbor in a friendly way, as it sounds like a wonderful dog who is just looking for some good company, and has figured out that you are a person he likes.