I wouldn't be so quick to cross of Lab from your list. They are extremely popular/common and they, like Pitties, tend to show up a lot in mixes.

Here's a pic of a Pittie/Lab mix for comparison:


If he were lab, that would explain his "chunky" legs. Boxers and Pitties have spindly legs. Labs tend to give their mixes "stocky" legs. But the best thing is to wait until he's full grown We could all be wrong!