Advice on an issue (Strong language)

So, to start off, I am a high school student. I came out publicly last year, and since then there has been this guy who just keeps flirting with me. He is very attractive, sweet, and caring, but I am just not into him. I've told him time and time again -- in a nice manner -- that I am not interested in him. He would nod and say he understands, but go right back to flirting and trying to get with me. At this point it's frustrating, and makes me uncomfortable. What should I say to not hurt his feelings, but get my point across? It's stressing me the fuck out and making me feel like an asshole.