I am being harassed by my ex husband even after mutual divorce. Please help!

Hello, I am a divorced female with a son of 4 year old age. I faced harassment(physical, mental and emotional abuse) during the marriage from my ex husband and his family, we had agreed to go with mutual divorce to make it civil for all of us. The harassment and ill-treatment went even worse from the date of first signature. *The divorce has been mutually signed and approved by the Judge, divorce document has reasons for divorce and how would mother(wife), son and father would live life after divorce, for example, mother will have full custody of her son, father has only limited visitation rights and also both would never file any case or do anything against each other and would have no objection for second marriage.* I am overdoing the visitation rights, the father visits along with his family takes my son and keeps with him in a hotel for 2 o 3 nights. They still expect phone/video call to talk to his son every day. If they don’t call, they expect me to call them. Now, he is asaking me, to send my son to his place for 15 days. Every time ex-husband talks to me, there is always shaming, disgust and condescending towards me. Now he has visited my uncle and has said that when I go for second marriage, he will take away my son and he will never marry again. Given the background, I want to be prepared and ready from legal side. Can he take away my son legally if marry again or in any circumstances? How should I handle/deal with him and his family?