Generally, if you and your spouse lived separately for at least the last 6 months of the year, then you should be able to claim Head of Household. The basic requirements for the filing status are that you are considered unmarried, paid more than half of the costs of keeping and maintaining your home, and that you are able to claim a dependent.
If you are not divorced by the end of the year and your spouse lived in your home at any time during the last six months of the year, then neither of you is eligible for any filing status other than married (either jointly or separately). If he is receiving some sort of financial aid and is using your address, the source of the aid may be advising you that you do not qualify because it appears that he is living in your home.
IRS.gov has several resources that list the conditions to be considered unmarried for tax purposes. These include the instructions for the Form 1040, Publication 17 (essentially the detailed instruction book for the 1040), and Publication 501 which focuses on dependents, standard deduction, and filing information.
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