He is an independent contractor whose pay in most cases is not taxed so he's gotta set some aside to pay when tax season rolls around; a lot of people cannot manage banking a third or more of their pay on their own and leaving it untouched. Wouldn't be surprised if Nick is this way, and I think this is related to a lot of rich people's tax problems. Nick needs a damn accountant.
That said, I don't think he's going to be doing any time. The IRS is not stupid- they don't want to put people in jail, where they can't earn money, and in fact cost money to charge, try, and incarcerate. Their goal is to get that money and reserve prosecution for the few that need to be made examples of, like active evaders who are not just ignoring filing (lots of people don't file regularly), but lying on their returns, ignoring demands to make payment, etc. He'll be racking up interest the whole time, but from his first contact he'll get a lot of chances to set up reasonable payment plans (which once again, he just needs a goddamn accountant to make sure actually get followed thru on).
Everybody good, plenty of slaves for my robot colony?