When you are legally married you are not Single for tax purposes, you can be considered "unmarried". You can be Head of Household if eligible.
See this TurboTax support FAQ for Head of Household when married - https://ttlc.intuit.com/turbotax-support/en-us/help-article/taxation/married-person-claim-head-house...
Since you are legally married, he would have to file as Married Filing Separately if you are filing as Head of Household. Unless he also has a qualifying child being claimed as a dependent on his tax return, then he could file as Head of Household.
