How to claim the custody of son from his father because of harassment?

I am divorced 4 years back having 2 kids, an elder son (14) and a daughter (9). I stay in India with my daughter. My husband stays in Dubai and has taken custody of my son. I have come to Dubai on a visit to see my son.

In these 4 years after my divorce, my ex-husband has tortured, threatened and beaten my child. My son now wants to come back to me, he is mentally sick. My husband has his passport and is not allowing me to take my son.

What should I do and whom should I get in touch with in the UAE?

I badly want my son, the impact of his father’s harassment is deteriorating my son's life. Please help me. Should I go to the police? Will anyone get me his passport?