If you are married and live together with your son, file a joint return, say no one can claim you as a dependent, then claim your son.  On the joint return, you will get credit for three exemptions: your husband, yourself, and your son.  You are not a dependent but you get a personal exemption.  If you don't live together, only the custodial parent can claim head of household (if they pay over half the household expense and if the non-custodial spouse didn't live there after June 30).