One other thing, we have to be careful to not put our own feelings in our dogs. What I mean is, the older dog walking away, and not being confrontational, is the way dogs are. Someone has to be the alpha, they both say the new dogs is. He is still growling at the old dog because someone there doesn't get that he's alpha and he still has to prove his place, could it be you? Sure to us people, that doesn't seem fair, but in dog-ese, fair doesn't count. If they are both happy with the new dog being the alpha, who are we to not go along with that?
Start treating the new dog like it is alpha - feed it first, pet it first. They are working out their hierarchy, it will only cause them more stress if you don't respect that. They will be much happier if you honor what they are working out.
They are going to be happier if they know their place, and you do too.
The older dog does not mind not being the alpha, as long as everyone in the pack knows it too. You are extremely lucky the old dog just walks away. The problems will start when you give signals that the old dog is trying for alpha, then there may be fights