Should i claim my son or should my fiance?

It depends.

It's unlikely that you would qualify as Head of Household, unless you:

1. Were unmarried as of December 31, 2016 and
2. Paid more than half the cost to run your home (or the home of a qualifying parent) in 2016 (rent, mortgage, utilities, etc.) and
3. Supported a qualifying person who is related to you by blood, adoption, or marriage.

The Qualifying Person rules are stricter than those for claiming a dependent. For example, although you may be able to claim a roommate as a dependent, that person would never qualify you for Head of Household status.

However, you may be able to claim an exemption for the child if he qualifies as a dependent. 

If all of the statements below are true, you can claim him as a dependent.

• Relationship: The person lives in your home for the entire year and is considered to be a member of your household. If they don't live with you, they need to be related to you.
• Income: Generally, their income is less than $4,050 (not including Social Security and welfare).
• Support: Generally, you provide more than half the person's support.
• Marital status: Generally, a dependent can't do their taxes with a spouse (married filing jointly). They also can't be a dependent on someone else's return.
• Nationality: The person is a United States citizen; or a resident or national of the U.S., Canada or Mexico.

If your fiance does not provide over half of the support for the child, she cannot claim him as a dependent on her separate return.