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Hello. Thanks for the question.
This may be a tumor, or it could be an abscess. Either way, I wouldn't mess around with it at home. I would just run him in and have the vet do a needle aspirate. If they come back with pus, then it's an abscess. If it's abnormal cells, then it's a tumor.
If it's a tumor, they'll likely remove it and biopsy it. If it's an abscess, they will likely drain it and flush it out, then put him on a course of antibiotics. Some abscesses are thick walled and hard. In that case, it may just be easier to remove the entire abscess and close up the normal skin. It really depends on the size, location and how it looks once it's opened up. No matter what, surgical treatment over topical is indicated.
I hope this helps, let me know if you have any other questions.