Unless Warren's son crushed his computer's hard drive, there will be a trail of all the websites he visited, along with all the searches he did and the links to all his forum postings. By default, the link to every single one of your forum postings is logged in your browser unless you deliberately turn it off. Even when deleted, they might be recoverable, unless care is taken to overwrite that data with new data (i.e. a zero fill operation).
Then I would think it would be easy to trace who sold him the gun.