I had a situation like that a few years ago. Thankfully, income was low enough that the repayment amount was limited, so she just paid it. But before we realized that, we talked about calling up the exchange and/or the health insurance company and getting them the retroactively rescind the policy and pay back the subsidy to the government. She never actually made the phone calls. But I figure it was a long shot. You might give it a try, if you think the savings would be worth the effort given the doubtfulness of the results.
Otherwise/most likely, I think he'll just have to pay the APTC repayment amount.