Thanks for that further information, and great description!
You are describing loss of pigment which is called depigmentation. When we see this without any other lesions such as crusting, scaling or ulcers, it is most likely something called vitiligo. With vitiligo there will be a complete absence of pigmentation along the lips and gums and sometimes around the eyes. The tissue will be nice and pink. No redness or ulcers.
Vitiligo is prevalent in Rottweilers and Doberman Pinschers, but we see it in other breeds as well. The disorder is characterized by non-inflammatory symmetrical depigmentation of the nose and lips primarily, but other areas may be affected as well. It is thought to be due to a hereditary immune-mediated destruction of melanocytes.
Affected animals should not be bred.
There is no effective treatment for this disorder.
Here is more information about this disorder:
So, there is no treatment, or way to prevent this from continuing until the whole lip area and perhaps nose become pink. If your boy is not neutered, you should keep in mind that this is not a trait that would be considered desirable and he should not be bred.
I hope that helps you. If this has been helpful, please hit the green "Accept" button and leave feedback. If you need more information, just click on reply and I will still be here to provide it!
The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.