The IRS gets copies of the investment tax forms (1099-INT, 1099-Div, 1099-B). If it adds up to enough to file, he will be hearing from the IRS. If he fails to respond (file a return), the IRS assumes their calculations are correct and will try to collect the tax and penalties.
!099-B's are particularly troublesome. The IRS will assume the gross proceeds from the stock/fund sale are taxable, not just the gain from the sale.
The filing requirement is $13,600, of income, not including social security, for someone over 64.
There are both late filing and late payment penalties of 5% of the tax owed amount for every month (or fraction thereof) your return is late, up to a maximum of 25%, each, in addition to the failure to file penalty.
See <a rel="nofollow" target="_blank" href="
https://www.irs.gov/irm/part20/irm_20-001-002r">https://www.irs.gov/irm/part20/irm_20-001-002r</a>
