I601 waiver

OK, so currently there are dozens of grounds of inadmissibility that an immigrant would apply for a waiver for. You did not indicate what ground he is deemed inadmissible by Uscis. Ultimately, if there is any ground of inadmissibility, a waiver would need to be filed and the burden is on you to prove that if the immigrant spouse was not allowed back in the country for permanent residency, then you would suffer extreme hardship.