I am sorry to hear you are worried about your Labrador. My name is Rebecca. I am a veterinarian and will do my best to help.
How long as the lump been there? Does it seem to hurt or bother him at all?
The most common soft lump we see in this breed is a benign fatty tumor called a lipoma. If it is painful, or if it is growing steadily, I would be more worried about something more serious like a mast cell tumor. If it stays the same size, does not bother him, and is moveable it is probably a lipoma and can be left alone.
The only way to know what any lump is would be to do FNA, fine needle aspiration. This means using a fine small needle to poke into the lump, to get some cells from inside it to examine under a microscope. If only fat is seen, I would not be worried. If any abnormal cells such as mast cells, are seen, I would have it removed as soon as possible.
Let me know what else I can answer. Kindly consider rating my answer so the website knows to credit me for helping you. You can still ask more questions and send me updates after giving a rating. This probably is not serious and Labradors are more prone to forming lipomas than other breeds, but it is alway best to be sure.