I read it as a 100% pitbull. He's not, so you're fine...unless he is aggressive and therefore someone could claim he falls under the "individual dogs that demonstrate..." sentence. Just say he's a boxer if anyone asks and don't make a big deal out of it...I think you'll be fine. And just remember, you may not always have a choice of whether you can live off base or not.
Secondly, what a fucking stupid policy...I know tons of people with every type of breed in that list and the dogs are great and not aggressive at all. It's jackass owners who have forced bad rep on these breeds.