If he isn't showing any obvious signs of distress and not fucking with it constantly, I'd monitor it for a few days and see if it gets any bigger or worse. Animals sometimes just get lumps for whatever reason, dogs more often I think. Since it isn't on someplace a little more vital, I think you can afford to just keep an eye on it at first. If no changes for the worse happen, just remember to bring it up at your next visit.
Of course, I am not a vet, so if Meow gets something horrible, don't blame me. Still, that's what I'd do for my own cat, up to you how much disposable income and time you have to spend on something that may or may not be anything to worry about.