However, I do have to agree that he certainly looks like a pit/boxer cross! And he may very well be. Have you seen the DNA kits you can get from the AKC? If you want to know for sure what he is, this is a really easy and relatively inexpensive way to find out. All you have to do is order the kit from the AKC and use the swab to collect a little saliva and skin cells from the inside of the dog's cheek. You send that off to the AKC and they send you back a certificate telling you what breeds and what percentages of those breeds are in his background. It's quite accurate!
Be careful with the DNA testing, they are far from accurate. Most vets/trainers/breeders (or at least the ones Ive talked to) say the test is a joke. I really looked into them a few years ago to see my dogs exact mix. Everyone I asked told me it was a waste of money.