If he dad to pay a fine, the max would be the average price of a bronze plan for his age. Maybe something like $5500 or so..
dad = had
If you have Medicare (age 65 and up), all you need is the "free" part of Medicare (Part A hospital coverage) to be Obamacare-compliant, and thus no penalty.
If you aren't Obamacare-compliant, the percent penalty applies to a "modified" income in excess of the standard deduction and exemptions (approx. $10,000 for an individual).
So as an approximation, it would be 1% of (your dad's adjusted gross income minus $10,000).