Well he doesn't look exactly as a boxer is meant to - the muzzle is pretty long and isn't apparently (obviously) undershot. His ears are very large too. But that doesn't necessarily mean he is a mix - there are plenty of supposedly purebred boxers on this site that are less characteristic of the breed than your boy. So I wouldn't take the shape of his muzzle as definitive by any means.
I do have a question about his colour though. It is difficult to tell from your photos, but is he brindle or is he black? If he does have definitive brindle striping, then the question is still open - he may or may not be purebred. If, however, he actually is black (or more correctly a very dark solid brown) then you can be certain that he is a mixed breed.
Purebred or mixed, he's a very cute puppy who'll undoubtably grow into a great dog. Love the photo of him with his mouth open and ears flying btw - what a cutie, enjoy him
