He cannot file as Head of Household using you. End of discussion on that.
If you were divorced in 2012 or earlier, he can claim you as a dependent under the Qualifying Relative rule even though you are not related. If you divorced in 2013 he cannot claim you as a dependent on his 2013 return.
You can claim an unrelated person as a dependent under the Qualifying Relative rule if ALL of the following conditions are met:
1. The person is a US citizen or resident of the US, Canada or Mexico.
2. The person is not the Qualifying Child of another taxpayer.
3. The person does not file a joint return with another taxpayer.
4. The person lived in your home for the entire tax year.
5. The person had less than the personal exemption amount ($3,900 for 2013) in gross income (excluding only non-taxable Social Security) for the entire year. Gross income includes all income from all sources before any deductions whatsoever, including normal business expense deductions.
6. The person received more than 50% of their total support from you for the entire year.
7. There is no state law or local law or ordinance that prohibits cohabitation. Any such law or ordinance, even if unenforced, kills the exemption. Mississippi, Virginia, West Virginia, Florida, North Dakota and Michigan still have laws that prohibit cohabitation as do a number of cities, towns and counties throughout the country.
If you can claim the person, you only get the exemption. No Head of Household filing status, no EIC, no Child Tax Credit, etc.
See IRS Pub 501 http://www.irs.gov/pub/irs-pdf/p501.pdf for more information.