What breed is this stray mutt? please help identify?

Puppy is about 2–3 months old, a rescue. Not mine.

He’s got

- floppy ears
– eyes only slightly wide-set
– thick snout; black nose and lips
– big-boned, big skull (similar to a labrador’s head? bear-like and triangular shape), thick paws
– probably will grow up to be medium or large in size
– short coat, brown and black color, with black toenails
– long pointy tail
– unafraid of people; actually look forward to being with people
– learned “sit” within 3 repeats

Here are his pictures:
without flash
with flash

Boxer? German Shepherd? Lab? We just don’t know.

Thanks very much for any help!