Agree with the consensus that a Chocolate lab was romancing somebody in the pedigree at some point, but he's a handsome boy, and if he wiggles and kisses, you've got the most important part!
I don't think your vet will have much to contribute on his purebred status -- as others have pointed out, he has traits that do not occur in boxers, so he's not a purebred boxer. Even if he had papers it wouldn't change anything.
As mixes go, lab/boxer is a pretty good one in general (See the Martha's Vineyard Black Dog, for example). You will have your hands full as he grows, though, because both are very energetic puppies (hopefully the lab side will kick in and he'll settle down before a boxer typically does

)
Enjoy him!
Kathryn Saxon
Daisax Boxers