You cannot file a joint return with someone else unless you're legally married to that person as of December 31st of the tax year.
So, if you and your fiancé get married by December 31, you can file a joint return for this year.
Second option: you could file as single and claim him as a dependent if he meets all these criteria: you lived together all year long, his gross income for the year was less than $4,150 (2018), and you provided more than half of his support. You must also live in a state in which cohabitation does not violate local law.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
