If he is signing up for insurance for 2021, the Premium Tax Credit for 2021 is based on his 2021 income, not his 2020 income.

If the NOL will be used up in 2021 and the taxpayer's income will be in the qualifying range, he will receive the Premium Tax Credit.

If you can show that projection to the health insurance person and that person is competent with such matters, your client may be able to receive the Advance credit to reduce his monthly payment.  If that person is not able to do it, your client may want to ask for his supervisor for assistance.  If the Marketplace still will not give the Advance credit, the full amount of credit will be received on his 2021 tax return.