He looks purebred to me, but no way to know for sure... he certainly has GSD phenotyping...
If you're new to the breed, you'll learn a lot here! Welcome to the community!
You can also read through the German Shepherd Guide, it also has some great information for people new to the breed!
German Shepherd Guide - Home
Does he have a name yet? Are you going to try to find his owners, or are you keeping him? I think its worth it to at least try to see if anyone is desperately looking for their lost puppy... if he's just a stray, congrats on the new puppy! Glad he found himself in a home instead of on the dangerous road.