I think a bitch of either breed could be a good match for your boy. First though, make sure he has gone through some good obedience classes with you, and has been neutered before bringing in a bitch.
You might want to wait till he is a little older because at his age they are quite the handful. You have to have a lot of energy, time, and patience to bring in a second pup or dog at this point. You might want to consider an older dog from rescue. At any rate, make sure that they meet on neutral ground and get along before introducing them at home. You will need to keep them seperated completely unless you are 100% there to supervise - especially if you get a young puppy. It is not unknown for a dog to kill a new puppy - so I am not saying this lightly!
If I were you, I would contact some good breeders and get their opinion. I don't know too much about the breeders in the UK, but I do know there are some wonderful Dobermans there (even with those floppy ears - LOL).