You can file as Married, Filing Separately, but no matter what you will need his Social Security Number if you wish to e-file your return. You can get his SSN from a copy of a previous return when you filed together, or request a copy of the return from the IRS here: https://www.irs.gov/taxtopics/tc156.html
If you can't get this information, you would have to file by mail and indicate that you don't have your spouse's SSN.
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