Has the lump appeared suddenly? Or grown gradually? Does it feel rounded and soft, or moves about when you feel it? Does it look at all reddened or inflamed? Does it feel attached to underlying tissues, or can you kind of 'lift the lump up' when you feel around it?
Lumps can be all kinds of things: reactions to stings/cysts/lipomas/abscesses/swollen lymph nodes/fatty tissue/cancer. A very high percentage of lumps are NOT cancer.
A lump that is movable, soft, doesn't feel attached, is most likely quite harmless and may not need to be removed unless it interferes with surrounding tissues, or becomes infected. My dog has a number of small lipomas which do not get bigger, and the vet said they don't have to be removed. She has had them for years, and they cause no trouble.
But if the lump is either: inflamed, very painful,it could be an abscess and will need to be drained and disinfected (professionally so infection doesn't spread from it) or feels hard, knobbly, and deep (attached underneath) it probably needs to come off as soon as possible.
With any lump it is better to check it out with the vet. That is always the wiest move, even though it may be perfectly harmless.
My 11 yr. old Dalmation has a good sized lump on left under-side, close to bottom ribcage. Vet said it's a fatty tumor which is nothing to worry about. He's also had several skin tumors that make nasty sores on knots at skin level. We've had them removed cause they looked so bad & seemed to irritate him but they were no threat to his health.
Hope this is same case with your dog! But if you're pretty concerned & can afford it, I'd take him to vet to make sure.
The only way to tell for sure is to have a vet look at it. It's impossible to tell what it could be simply from your description of it.
Dogs do develop lipomas (fatty tumors) in the area that your dog has the lump, but it could also be something else, so it's best to be safe and have your vet check it out. He may or may not want to do a biopsy to make sure that whatever he thinks it is is correct.
My dog has fatty tumors that are harmless. They are soft and kind of move a little. BUT she did have a small lump on her leg that was a Stage 2 mast cell tumor (cancerous), which can kill rather quickly. It was not painful, just a little smooth lump. It's best to have any lump checked. The vet removed the cancerous tumor with just something to numb the pain (a local). My dog was very lucky.