"he was automatically enrolled to the health coverage" - this is puzzling because no one gets automatically enrolled in a marketplace policy. You probably want to find out how this happened.
Since you are claiming him as your dependent, you have created a Family unit, and the income of all Family members is added together to determine the expected contribution amount that your family would be expected to contribute towards the insurance premiums.
The reason that it's called the "Affordable Care Act" is because when you call the marketplace, you tell them what you expected income will be. The purpose is to determine how much the government will contribute to your insurance premiums.
I am guessing that your dependent said "zero", which meant that you got the maximum premium tax credit (payment/subsidy towards the insurance premiums). But as Dawn said above, once your return is done with the two of you together with your hefty income, it turns out you were not eligible for such a big subsidy, so you have to pay it back.
So, immediately, call the marketplace and tell them how much the two of you together will make so that they can adjust the subsidy you are getting.
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