I think he may have some border collie too cuz of the color. And maybe rat terrier cuz he is "smedium" in size. I love the "thumb spot" on the top of his head, my buddy's border collie has that and he's super duper awesome (I think it may be an indicator, a lot of the "thumb spot" dogs I've been around have been super intelligent). Whenever I say border collie he tilts his head harder than most other words. His favorite word is football, he'll tilt his head really hard for that one lol.
Here's a heeler (australian cattle dog) x rat terrier. note the squeaky love lol
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when groomed my old boy has that same mottled look. if I can find some pics I'll share.