My MIL always interferes whenever I discipline my son. As a result, my son always runs to her whenever I am disciplining him and this has made him defiant. How to tackle this situation?

i think you should immediately talk to your MIL about this and tell her how her behaviour is creating a negative impact on your son. tell her that as he grows up, he will look up at his grandmother as a role model, but will realize that what she is doing is not correct. and this will especially hold true when he is reprimanded by others for his misbehaviour. if she absolutely fails to see reason, please get your husband involved and make sure he understands your concern.