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OH, OK, thank you for the clarification on that. You mention that there is an infection on the gum -- could you explain a little more about where on the gums this infection is? Upper or lower? Near the front of the mouth by the canine teeth ("fang" teeth), or in the back by the molars?
Is the are that is supposedly infected just red and irritated, or is there any new tissue growing, like a tumor might look?
When he is on the antibiotics, does the area LOOK normal again? Or does it still look infected/diseased but stops smelling?
If the smell is coming back while still on the antibiotics, then there must be more going on than just an infection. In dogs with very droopy lip folds as you have described, it is possible to develop a dermatitis, though it's not very common in Cockers -- usually Newfoundlands, St. Bernards, Mastiffs, etc are more likely to have the problem associated with excessive lip folds.
If your veterinarian thinks that surgery would be helpful, I would be inclined to agree, based on the information you've told me so far. If the mouth is otherwise healthy, then perhaps removing some of those excessive folds would be a good idea.