Establishing liability seems easy. Winning a judgment in small claims court ought to be straightforward. Actually collecting on that judgment could be a challenge depending on where the company is located and how they do business.
Honestly, I don't think he'd need a case against Amazon for this. If the original company refuses to pony up and Amazon was made aware, I feel like they'd just do it since it would be good PR to help make it right since what he's doing is a huge service for Amazon's customers.