I'd be willing to bet money he's not 100% Boxer based on the head alone. He's got a longish muzzle, there's no upturn to it at all and his stop slopes too much. Therefore, the planes of his muzzle and forehead are incorrect for the breed. The first thing lost on a boxer when outbred to another breed is usually the head and muzzle. Also his black markings don't extend very far up his muzzle into much of a mask which alone doesn't mean anything but paired with the incorrect head make me say "mix".