My boy best friend refuses to stop contacting me. What should I do?

I have a friend who just started high school, where he transfered a few months ago. He's been texting me very often, trying to keep in touch with me and reach out to me.

I've always been extremely introverted, and socializing (either texting, calling or meeting) a lot is not my thing.

A few weeks ago, he confessed that he's in love with me. I told him that I didn't feel the same, but ever since his confession, it feels like he's been contacting me even more. He texts me 24/7, and if I don't reply within the next few hours, he texts me something like: "answer!" He's also showing up by the bus stop when I'm on my way home from school. And, if I don't feel like sitting next to him on the bus, he gets mad and runs away.

I've told him multiple times that I usually don't have the energy to meet with him or reply to his texts, but he doesn't understand why. Sometimes, he gets so upset that I need to make up excuses so I don't hurt his feelings. It feels like the only way to make him accept a 'no'. But, if he realizes that I was making an excuse, he gets angry and says that I'm 'avoiding' him.

I feel like he doesn't respect my boundaries. What should I do?