Creepy guy told me he does gross things while looking at pics of me and then he kind of started stalking me?

[lola]

So I was at the mall today waiting for my friends and this guy I know from school came up to me and said: "hey lola, wanna know something?" And was like sure what ever. And then he told me he likes to masturbate while looking at photos of me in bikinis on Facebook... I was really creeped and told him he was a perv then walked away. Then me and my friends were in the changing rooms at a store and I walked out to look in the mirror and he was standing there with his hands down his pants and he was so hard I could see it through his jeans. I'm super freaked out because I have a bf now to and I don't know what to do! Should I tell anyone? I'm also worried that if my big brother finds out he'll beat the guy up... Plz help!