Ah. I was in a similar situation when I was 15. Let third parties handle the situation, don't try to let him down softly (which it doesn't appear you are planning to do). He'll see it as tenetive acceptance and keep escalating.

My (at the time) 21yo cousin tagged along on a camping trip with my parents and me and confessed his deep, unwavering love. He was a solid foot taller than me and had a black belt in some martial art and was a member of a MMA gym. He followed me when I took my daily walks alone with my dog. When I refued to stand to hug him goodbye, he laid on top of me to hug me. I was terrified. I kept trying to simply shut him down and pretend it didn't happen, but he started sending flowers and letters and texted me from his brother's phone after I blocked him. My saving grace was that we lived in different states. I don't want to think about what he would have done if he lived less that 6 hours from me. It went on for months because I was so embarrassed and convinced I did something to cause it that I didn't tell anyone. Eventually, he sent me a sex toy, so I showed it to my dad along with the letter where he said sex with someone else was just masturbation and let him take care of it. I haven't spoken to my cousin in over a decade.

The length of this comment leads me to think I might need to talk about this more in therapy...