a 6.5 month old amazing GSD. His temperament is amazing and very stable. He is BIG (comes in at 84.5 lbs already) and he is VERY strong already. He is super smart as just within the first week of being with him I have been able to train him to consistently follow SIT, DOWN, STAY, PLAY DEAD, LEAVE IT, CRATE, etc.
It's great!!! You've got a true GSD from Working Line
, and it means, that you've got a road to success redcarpeted. Working line GSDs you can train absolutely everything, you can make them agressive, but you also can make them incredibly docile and amiable. Keep on consulting the professionals on this Forum on every step of this matter while watching your rabbit and your dog' relationship in progress.
So, how you describe, they are two males. Dogs recognise sexes intuitively, they know who of us is a man, and who is the woman. They know smelly cats and stinky ferrets, they know fouling pigs and sounding horses, they know their sex. So, one of your males is an intruder in your house. It couldn't be otherwise between two young males. Your dog wouldn't react on your rabbit as a preditor reacts on his natural prey, because he was never taught that the rabbit and his smell could promise a meal. He, the intruder, can react on the rabbit as one male react on presence of the other, and dog males at the age of 6 months like to engage in a rough play when they see any possible companion. The intruder always behaves agressively, he has to, and your task is - not to suppress his agression, but redirect it.
To stop both males competing for territory you should separate them by providing a shelter where the smell of the other could be lesser detected. It could be nice to leave your dog outside for night, night smells and sounds will take his attention away from the worried rabbit.
Sometimes, rabbits behave agressively towards house dogs. They chase them round the back yard trying to bite, really pushing that barking bit out of their territory, because male rabbits, as any other males are territorial. You might think you know your rabbit, but he might prove you wrong. The tables might swap in the way the least expected, you might train your dog to be gentle, but your rabbit might start taking advantage, attacking your dog, dog's instincts reviving, and - again, very unpleasant conflict, inside out conflict.
IMHO, you should teach them both that they have their own territory, and their own place to sleep, and their own plate to eat. Meet them outside on the grass more often, playing ball with your dog (leashed) and teaching him not to do any movement towards the rabbit, it should be a forbidden toy for him. Ask him to lie down every time he pulls toward the rabbit. These sessions gradually could become walks together in the park with your rabbit and your dog unleashed. I'd suggest for inside the house training to teach your dog to be patient at command "Sit!", he must keep sitting whilst your rabbit hops around or sits on your wife's lap. For a short while first, then make it longer, finally, doesn't matter what your GSD feels towards your bunny - he will sit, lie down and just watch him. They might hardly stand each other in the future, but be optimistic, always tell your dog in human words that you love him more than that fluffy thing, all males need to be told it. Sure, you will tell your rabbit absolutely the same.