I fiance and I have been living together for the last 2 years. I'm thinking of including him on my taxes since I make more than him and pay most bills. How do I do this?

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.

say you do get married by 12/31.  he still has to consent to the filing of a joint return.  otherwise both would have to file as married filing separate.

one more twist.  a few states recognize common law marriages.   However, each of these states have specific criteria that must be met for it to be recognized.

for example - in Utah - a written agreement between the parties.   certain other criteria  that must be met.  finally going to court to have the "marriage" validated