You can't file a joint return unless you're legally married. He could claim you as a dependent if you meet all of these conditions:
1. You're not a Qualifying Child of another taxpayer.
2. You had gross income of less than $4050 in 2016
3. You and he lived together the entire year of 2016
4. He provided over half of your support in 2016
5. He can't be claimed as a dependent on someone else's tax return.