No, there is no way to find out if he is a purebred boxer. Certainly not from a microchip. If you happened to have access to the dogs named as his parents on his pedigree, you could determine whether or not they are indeed his parents. But you still can't determine breed.
As for German breeders breeding large - not if they're breeding correct
The German (actually FCI) breed standard stipulates height also. And if one were to make a generalisation about size, then one would also have to say that (broadly speaking) boxers bred in Germany and most other places in Europe are less
tall than many in North America. Frequently they have more "bone" - as in, are often more solid or stocky - but they are not normally taller.