I agree with AngelBunny....most dogs do not mind being alone, and the # 1 dog.
They often get more time and attention then they used to get when there were two dogs...no need for sharing.
If you feel you need and want another dog, get it for yourself...not for Junior.
A female dog would be the best choice, and of course a rescue dog would be even better. Nice if you could save a life and adopt a dog from a shelter or a rescue.
I think Labs and Rotts make good companions. Probably a Lab a bit younger than your Junior...so it would motivate him to play.
We have a long-term boarding Labrador staying with us right now...and he get's along great with both of my dogs....and they even play together.
Gina