The problem is that he probably did not know it was even stolen. I would call the number, see what they say. They are a current company, that is a current # and website, and a current logo, so it's not that old of a keg.
I would call the number and see what they say. it's the right thing to do. If they say keep it, then kudos, you have a keg, but without proof, it's a hard sell.