How do I stop being afraid to fight back or talk back to people at school

I just want to know how I can handle this at school. There’s this guy who has fought me twice already, and both times I didn’t do anything when he hit me because I was scared. It’s not that I’m small or completely clueless about fighting. I mean, I don’t necessarily know how to fight well, but my dad boxed and has taught me some basic stuff, so I know the fundamentals. I also have a pretty big build for my age, and that kid is smaller than me. He’s also in a gang so maybe that's a bit why I am afraid and I also know I can fight better then him but for some reason don't do anything when he hits me