Hello and welcome to Just Answer. I am a licensed veterinarian and would be happy to answer your questions.
Is it an upper lip or a lower lip? If lower, is it on the lip itself, or more on the chin?
Is there any drainage coming from it?
Does it seem painful?
Is Buster an indoor only cat or is he allowed outdoors?
Does it resemble either of these pictures?
What Buster is likely experiencing is a condition known as an eosinophilic granuloma. They are an inflammatory reaction thought to have an allergic cause but often the cause of the allergy is not identified. In most cases, they need a treatment (or two) of steroids to resolve it and if it becomes a recurrent problem, further testing may be needed to better identify the allergen.
If that is what it is, it is relatively non-painful, but can be irritating enough to cause him to rub it. You should have him evaluated by your veterinarian. In the meantime, it would not be harmful to give him a dose of benedryl, but in most cases, steroids are need to reduce the swelling. The dose is 1 mg per pound so a 10 lb cat would get 10 mg. This can be repeated two to three times per day. The pill that is available over the counter is typically 25 mg. so you would probably need to go with the children's liquid form which is typically 12.5 mg/teaspoon (or 2.5 mg/cc). Make sure that there are no other "cold-type" medications in it like decongestants.
I hope that helps. Here is a link with more information: eosinophilic granuloma
Please let me know if you have any further questions.