Having DDBs, I agree with the stubborn streak. They can be willful buggers! But you must be too!
Calm, confident, DOMINANT alpha. What does that mean?
It means to practice certain things everyday in addition to training:
You sleep on the bed, the dog sleeps next to or below you.
You walk through doorways first.
You eat first.
Dog must sit calmly before food is placed on the floor.
Play fetch instead of tug.
There are many more. But in order to be alpha to your pack, you must act like one. Think calm but firm 'no' for misbehavior. And plenty of praise when he acts submissive or does what you want. Anger does not work with these dogues.
I would find a good trainer now before Zeus gets too big to handle.
I would also wait on the playmate...until you have Zeus's issues worked out. Otherwise, you could end up having two pups with issues. Pups can influence each other's behavior. A dominant pup can terrorize a more submissive pup...
We have a litter of six DDBs (all pre-sold) and at four weeks they are already establishing dominance over each other. I step in when necessary with a single finger to the side of the neck and a "SSST!" If the pup being dominant continues, I gently roll him over and hold him until he relaxes. No violence, no anger, no frustration. Just a simple correction that mommy dog would give.
Good luck with Zeus, he is a cute boy.