Voting regulation is not a power expressed to Congress (except as related to ensuring no bias in race, age, and gender), so his unilateral action has no basis in law. But, when has a basis in law ever applied to him? Lawful grounds such as ‘consent’ has never applied to him before. Why would it now?
For states that have had all-mail-in voting for decades now, like Oregon and Washington, this is basically wholesale disenfranchisement of the whole state.
We’ll just count the ballots as is. He can’t enforce anything.