Very hard to know or guess really. I would be inclined, like you, to think that there is something else there though. I cannot tell his size of course, but from the photos you posted I would remark that he seems to have an extremely long muzzle. Not a little long like you commonly see in boxer/lab or boxer/pit mixes, but quite substantially long. Possibly that could suggest that whatever else is there is a breed with a very long muzzle.
LOL - that's just guessing though (as is always the case when going on looks only), and genetics can produce some unusual/unexpected results. If you really wanted to know, it is possible to have him dna profiled