Is it outside his mouth or inside on his lip? Both have been shown on this board. If outside it could be an ingrown hair and will probably go away in a few days. If inside the lip then it should probably be looked at by a vet.
Maybe it's this? Puff had some on her lips when she was younger and recently had one removed from her left eyelid.
ORAL PAPILLOMATOSIS (WARTS IN THE MOUTH)
"Oral papillomas are painless warts that grow on the lips and in the mouths of dogs younger than two years old. They are caused by the canine oral papilloma virus. Initially, papillomas are small and pink. Over four to six weeks they increase in size and assume a rough, grayish white, cauliflowerlike appearance. As many as 50 to 100 papillomas may be present. Skin papillomas caused by the same virus are common and occur on the surface of the eyelids and the skin of the body.
"Treatment: Oral papillomas usually disappear spontanously in 6 to 12 weeks. If they fail to do so, they can be removed by surgery, freezing, or electrocautery. Chemotherapy is effective in the case of numerous lesions. The dog's immune system makes antibodies that prevent reinfection."
-- from Giffin & Carlson, "Dog Owner's Home Veterinary Handbook" 3rd Edition, 2000, pp 214-215.
(This book saves its cost many times over in avoiding unneeded vet appointments as well as unfortunate delays in seeing a vet. All dogs should come equipped with this book, all owners should consult it as needed. Less than $25 from Amazon.)
Mocha had acne when she was younger too. I found this picture while searching for what they were. Everyone told me to change to stainless steel bowls that a lot of times it's from bacteria from plastic water or food bowls. I switched and they went away.
It's still there. It has changed from pinkish to Mar's skin colour (like a greyish blackish). It still looks like I a pimple, I guess, and the vet didn't know what it was. I don't think it's acne as there is only one.
I'm thinking it may be an Oral papilloma, as Bob Pr. suggested. I'm going to keep an eye on it. It doesn't seem to be causing pain or discomfort.