In an older guinea pig such as yours, I would be most concerned about a subcutaneous tumor or abscess. How long has the lump been there, and has it grown at all? Is your g.pig acting normally? How’s his appetite? Any weight loss? If the lump does not grow, and he is acting fine, I would leave it be. If on the other hand he is acting sick, or the lump is growing, you will need to have it aspirated to rule out an abscess, or malignant tumor.