I was 'that guy' back in the day. I had known this one girl since I was 6. I didn't really think of her as more than a friend until I hit puberty. At that point I was just obsessed with the idea of getting together with her but I was too shy to ever really make a move on any girls as I had really bad acne as a teen.

Lucky for me, she left to Australia as an exchange student. That was the one thing that let me get over my obsession. It lasted for quite some time.

Then she came back home a few years later and at that point I learned that she was a very good friend of my brother's girlfriend. Over the next few years she had asked my brother's gf several times how I was doing in life which I found really weird as she had never really shown any interest like that in me before. After a while, thinking back, I realized that I had completely missed her signals and she had probably expected me to make a move on her when we were younger. I realized that I had fucked up and she was now married. My obsession re-surfaced and I knew I couldn't get her anymore. I did move on with my life but she was still the ideal girl in my mind and I have only ever been with very petite girls as a result.

My story is a bit different than yours but all I'm saying is that he probably has had an unhealthy obsession with you, that he can't get over, longer than you think. You might see him as just a friend but he'll probably always see you as someone to pursue. You need to pull the plug and force him to get over his obsession.