My husband purposefully took our sons passport with him to Africa.?
Verbally ask him to return back the passport of your son to you by post or courier. If hes refusing to do it, lodge FIR with the police for theft of the passport of your son and name your husband as accused. Mention his details, his passport details, address where hes presently living in Africa in the complaint with the police. It will be for police to recover the passport from your husband. Always remember, the passport of any person is not his or her property but its the property of the country which issue it to the person, that person is holder of the passport only. If the passport of your son is issued by the Indian authority, the theft of the passport is theft of the property of the Indian government and that can be big trouble for your husband, just tell him this fact.