Actually it is not married to claim Head of Household or lived apart from your spouse. Is he the father of your son? To claim Head of Household he has to have a blood relative.
Who is a qualifying person for Head of Household
https://ttlc.intuit.com/community/family/help/what-is-a-qualifying-person-for-head-of-household/00/2...
And is he claiming both your son and you as dependents? He can probably claim both of you if you lived with him all year.
