It would mean he loses his ACA credit on his tax return. The tax return reconciles the credit he received in advance and the credit he should receive based on his income.
So in short, it IS a very big deal.
If the actual income at the end of the year falls below 100% FPL you still get the PTC credits, as long as they accepted the initial estimate of it being over 100%.
...see the instructions for Line 6 in
https://www.irs.gov/pub/irs-pdf/i8962.pdf.
"Household income below 100% of the federal poverty line.
If the amount on line 5 is less than 100%, you can take the PTC if
you meet the requirements under
Estimated household income
at least 100% of the federal poverty line
next or
Alien lawfully
present in the United States
below.
Estimated household income at least 100% of the federal
poverty line.
You may qualify for the PTC if your household
income is less than 100% of the federal poverty line and you
meet all of the following requirements.
You or an individual in your tax family enrolled in a qualified
health plan through a Marketplace.
The Marketplace estimated at the time of enrollment that your
household income would be at least 100% but not more than
400% of the federal poverty line for your family size for 2017.
APTC was paid for the coverage for one or more months
during 2017."