I think Sysinfo and TheophileEscargot were right:
He's a Lurcher.
→[More:] At about 4 and a half months, Winston is up to 36 pounds. He's got a very wiry build which backs the greyhound theory. And, of course, he's got that shepherd mutt face. His sight hound characteristics are apparent; he is very attuned to sounds and movement. He's also a natural guard dog and most of the time (unless his puppy playfulness has him hyped up) he is a gentle giant. Our beagle can now walk right under him without hardly touching his belly.