If a person's income is less than the amount required to file, then no return is due. For a single person who is not someone else's dependent, that amount is around $10,000 per year (it changes each year due to inflation.) So people can sometimes have a legitimate reason to file one year and not another year.
On the other hand, a person is required by law to report all their income and pay appropriate income tax, even if their income is earned from illegal activity.
I can't answer what would happen if this person files a tax return for 2016. It depends on the overall situation. The IRS may simply say, "good, he's earning money and paying tax again." I don't think filing a tax return would trigger an investigation into previous years just by itself, but there could be other factors involved. If he has child support debts that have been reported to the federal government for collection, the IRS would probably withhold any refund he was due to pay those debts. And if he was ever to come under criminal investigation for his other illegal activities, that would include an investigation of bank accounts, money laundering and cash transactions, which could be used to raise a new level of criminal charges on top of the drug charges he would already face.
