Rather than too innocent, I would say let down by his elders.

It may have changed by now (though I doubt it) but this kind of thing was not covered in my high school or college education and nobody else taught me anything about how taxes work, and so when I entered the workforce in my 20s I was subject to a crash course on the subject after I wound up with a surprise bill from the IRS.

Given that he entered the industry at a young age he very well may have not had anybody teach him any better which enabled the accountant to take advantage of him.