Help?

Theres this guy at my school and hes in my grade & hes in 2 of my classes and he always talks to me. When the campana rings and its passing time he follows me to my locker and flirts with me. He really likes me but I have a boyfriend, but hes in 11th grade which is one grade ahead of me and it takes him like 3 mins to get his stuff for his siguiente class then come to my locker& the guy is there for like a long time and i try walking away but he follows me.. He tells me how cute&sweet i am and its just awkward.. So what im asking is how can I tell him that I have a boyfriend & that if my boyfriend sees him at my locker flirting with me , that he will be real mad& will literally like tackle him?