I don't see any "pit bull", boxer, or any other bully breed in that dog. I see some hound thought.
than why is his head so big?
Many, many, many breeds have big heads. In fact, "pit bulls" aren't supposed to have monstrous heads. Hell, my Shar Pei mix's head is larger than any of my APBTs. And not all hounds are 15lb beagles. However, he could have some Mastiff, Rottweiler, Shepherd, Collie, Rhodesian Ridgeback, Labrador (which may explain the blocky type head & large flat ears.....I've seen many Lab/Shepherd mixes that look like that dog)
If you don't see hound, that's ok. But I honestly don't think this dog is a mix of 2 purebred dogs, instead I think it's a mix of several different breeds & mixes thereof & we will probably never know what kind of heritage this dog has. However, wanting him to be a "pit" & knowing that he actually is are 2 different things. And IMO, if you don't know what the hell it is, don't go around calling it a "pit bull".