Dealing with a bully?

I'm a girl and there is this boy who cannot stop insulting me and making fun oh everything I say. he's always embarrassing me on purpose by calling me fat and saying my face is ugly and stuff like that. I am not afraid of him because we've known each other since year 1. its really hurtful though and I hate getting embarrassed. whenever I go over to my group of classmates and he'd be there too, he'd whisper loudly to people left and right saying something like "she's such a loner, keep ignoring her." I don't get why he's so mean. I asked him that a few times and he went "I don't CARE about people's 'feelings'." I'm planning on slapping him next time he insults me really badly. is that a right thing to do?