If he is required to file a Married Filing Separately tax return because the two of you lived apart for the entire last 6 months of 2019 and you filed as Head of Household, then he must file it to qualify for the stimulus. If you did not live apart for the entire last 6 months, you are not allowed to file Head of Household. If he is not required to file a tax return because he had little or no income, then he can choose the Single option on the registration form to register for the stimulus. Here is a link: https://www.irs.gov/coronavirus/economic-impact-payments