Maybe look at it as doing him a favor. People who need to put other people down in order to feel better about themselves seldom have many friends in life and a good friend helps a friend learn even unpleasant truths about themselves in order to improve their lives. Giving your friend a heads up about his bad habit and the likely results of continuing to do it should be worth the risk of losing the friendship to you because if you don't draw this boundary now you won't have a friend worth having and the relationship will deteriorate into an increasingly abusive one anyway. People who get away with small cruelties inevitably graduate to larger ones so you would probably be better off losing this one before you are so beaten down you are not even capable of making real friends with other people. Your choice if course. Good luck.