Its very possible he is a Dane mix. Danes and boxers are very similar in structure, and I have seen Dane/Boxer mixes that I could have sworn were huge pure Boxers as well as some that looked like very small Danes. And many unsavory breeders will sell mixes as purebreeds...
Another thought is that some boxer breeders (again in the unsavory category) do intentionally breed their boxers larger and larger. You might have just gotten your hands on one of them. I have heard of boxers reaching above 100 lbs, pure boxer, just very very poorly bred.
Either way, I'm sure you have a wonderful boy on your hands, and I LOVE the name Winston!

How did you come up with it?