A guy went from treating me like his little sister to wanting to have sex with me, help?

Please read what happened to me, I don't know how male brain works but I want to try to understand-
I met a guy when I was 16 he was 21, we were such good friends. We spent so much time together for a week (reading a lot, talking, eating, watching movies, we pretty much lived together for a week) I considered him my very good friend by the time I had to leave. After a week I texted him and we had a nice friendly conversation. He was always very caring and considerate with me, like I am his little sister, or at least a dear friend, he was to me. We live far away so in a year he texted me, I told him my 18th birthday is coming very soon and suddenly he started to be pervy. He asked if I have a picture of me in a bikini, what I wish to do if he was there, if I get "excited" when I ever think of him (excited is the best word I could think of in English, he basically used a proper way to ask me if I get turned on) I tried to keep that conversation as friendly as I could but it ended quickly, if this information helps it happened at 3 am, had to sleep. Then in a year I texted him, thinking this time everything will be fine, maybe he was just horny the last time and didn't mean it. But he was pervy again-He asked if I ever slept with a guy, what I want to do when we see each other again (I moved to a city where he lives), he said he wants something but considers I'm still not ready for it (like it's obviously sex isn't it lol) but when I started to get annoyed he tried to be kind again. Why did he go from treating me like a little sister, to treating me like he just wants to have sex with me? I didn't send him a bikini pic, I didn't say anything naughty to him either of times... Did you ever do this to a girl and why? Can I keep our friendship? Could he want a relationship? Could the first year of knowing him be just an act and this is his true intention? Tell me your opinion.
A guy went from treating me like his little sister to wanting to have sex with me, help?
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