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At his age there are two likely things this could be- the first is a anal abscess (infection) and the second is a skin growth such as either a cyst or benign tumor such as a 'histiocytoma'.
If it's an abscess then it will probably look red and sore and possibly will discharge pussy material. If it's a skin growth, then it's unlikely to look sore.
If your vet cannot tell what it is just by looking at it then i would advise doing an 'FNA'- fine needle aspirate, this is where a needle is inserted into the lump and a sample of cells removed for examination- that can be diagnostic in a lot of cases, and not very invasive, so is a good test to do first.