I wanted to add a couple things. First of all, he has not had any injections or or even near that area (I give him all his vaccinations, except rabies, and I always put them between the shoulder blades). And as far as the owner knows, he wasn't stung by anything either.
And the lump looks identical to a sub-q injection. That typical bump if you gave one. Looks the same and feels the same.
He's also not missing any hair from around the area and the skin is normal in color--not red in color or lumpy at all.