Let's assume that he only made half of the $20-40K mentioned from signing his gear. Assuming he is still classified as a dependent (not sure), I believe the minimum annual amount for earnings without filing a tax return is $5,700. It seems plausible that he exceeded that amount in any one of his three years at school so I assume there has to be a tax bill due.
I have not read anything regarding this angle, but the IRS is one organization that has ways to uncover whatever paperwork they want, particularly since Dennis Talbott, his partner in crime is also being pursued for unpaid taxes (http://sports.espn.go.com/ncf/news/story?id=6637444). I could only hope this helps the NCAA if they do not get enough from the checks (!) written to TP from Talbott.