Penalty for Underpayment

Taxpayer is retired and has income from social security and investment income. He also receives premium assistance from his ACA health insurance. He has no withholding and does not file estimated taxes. His overall tax bill (the amount he owes IRS from his 1040) is over $1000 due (in part) because he mis-estimated his income in originally signing up for ACA insurance. In other words, he received more premium assistance during the year than he was entitled to. So the excess amount he received is added to his tax bill on the 1040. In the absence of the premium adjustment, his tax bill would have been under $1000. Does he owe a penalty?