to me it sounds like you have a potentially, very good working dog, this play fighting is prey drive, I dont know if working him is what you want to do with him but if you dont he will do what he has to, to work his aggression out, I would start some rag work with him and do some mild bite work with him as well, to some this sounds crazy to teach a dog to bite but we must remember these are the only dogs that were Bred for the purpose of personal protection and even though many AKC show prancing breeders are trying to breed any working ability out of them sometimes Good Dobermans are still being born with that drive to work, I would really embrace his natural abilities and start some obedience and see what this little guy can do, he may impress you someday.