A married person can file as head of household (rather than married-filing-seperate) if he is considered an "abandoned spouse" for tax purposes. To be an "abandoned spouse" you need to meet these requirements:
1. You file a separate return
2. You maintain a home where the child for whom you may CLAIM AS DEPENDENCY DEDUCTION lives for more than half of the year
3. You must pay more than half the cost of maintaining that home, and
4. During the lat 6 monts of the year, your spouse must not live in the same house as you.
To claim the dependency, in turn, you must pay MORE THAN HALF THE COST of maintaining the child OR have an agreement approved by a divorce court that says you get the dependency exemption.