My dog has a soft, quickly appearing lump with no pain. What to do?

Has he been licking at it? Can you identify a wound on it? It may be an abscess (localized infection) under the skin, as thise can show up very quickly. You can shave the hair over the top and clean it with dilute Betadine (available at any pharmacy - use water to dilute it so it looks like tea). Put an ecollar (cone) on him to prevent him from biting or chewing on it, and make an appointment with your vet as soon as you can—but it’s not something you need to see an emergency vet for as long as it’s not seeming painful and he’s eating and acting normally. Other possible causes would be some type of tumor (though less likely with his age), or a fluid accumulation called a seroma. Your vet may Do
a fine needle aspirate (poke the bump with a needle and then look at cells under the microscope) to see what it is and then know how best to treat it.
Thank you for asking Petco Pet Education Center, formerly Petcoach!