It could also be feline acne. A lot of cats are prone to it and they get it in places that they have trouble keeping clean, like their lips or even their noses.
Usually it isn't serious. I'd keep an eye on it and if it seems painful or itchy or if it gets significantly worse, take him to a vet. Otherwise, like with people, it can come and go and generally isn't a big issue (unless kitty is trying to get a date to the prom:)
Feline acne shows up on light colored cats, like orange tabbies or white kitties, so we notice it more on them.
What your cat is experiencing is pretty common among orange tabby kitties. It can just be an odd pigmentation thing with orange cats, some people call them “age spots”. Concerned about this, some people have taken their cats to the vet. Answers have not varied so far: Nothing serious.