My husband a I separated 11/2010 with one child between us. Is it legal to file my taxes claiming my son and married filing separate?

Any married couple can file separately.
One parent gets to declare the child.
If there is a custody agreement, the custodial parent declares the child, unless the other parent has a signed agreement from the custodial parent attached to the tax form.
If there is no agreement, then the parent whose house the child spent nights at the most after the separation.
If equal nights, then the parent with the highest income.
For more details, get the tax publication at a PO, or go online for Support Test for Children of Divorced or Separated Parents.