You would need to obtain a certified export certificate from the Munich boxer club, which would then enable the dog's pedigree to be accepted by the AKC. There is information about what you need to do here:
http://www.akc.org/rules/special_registry_services.cfm
As a side note though, have you shown your dog and had any health screening done? Aside from being the only ethical and responsible way to determine whether an animal is fit for breeding, it's a requirement before permission to breed a dog in Germany will be given. Germany takes the responsible breeding of dogs seriously

Without that, I'm not sure that you would get an unrestricted registration certificate that would enable the dog to be registered in the US.