Your first mistake was trying to reason with a fourteen year old boy or girl.
Using this feminist language
“Do you understand that using that word in that spirit is very harmful to women? It’s degrading and disrespectful?”
was lost on him as well.
Adults have an absolute right and need to correct children for obvious rude behavior. This boy was trying to get attention by using a provocative word. He got your full attention. Just like any other rude behavior it needs to be corrected and not repeated.
By the way, fighting misogyny with misandry isn't the answer either. Boys and men deserve respect too.