Why is he like this?

So im dating this boy and I overheard him say hes only dating me to make me feel special, I got mad and he wrote me the sweetest poem, and was like im sorry then he turns around and says im dating someone else. My best friend says thats hes a lying sack of crap who is using me. But me being me takes him back, I’ve only been dating him for like a week and a half and he already asked me if I would marry him and that he wants to spend the rest of his life with me, but then he turns around and says that, oh and the good part about this all is that when I told him he lied to me straight to my face. I was kinda expected it since I heard it but stillhes my boyfriend. Then in the mourning after “the fight” he gave me a two and a half page note saying that he loves me and that he wants me for life and stuff, I mean its better than just a sentence like usual but I just don’t know. Prom is in a couple of days and he asked me to it but then he started talkig about haveing sex and stuff that makes me wonder if hes using me. I know I want him really bad but I just don’t know everyone says he is but you know what if a guy paid attention to you like he has you would see. Im open for any feed back even if its negative I really just want to know. Wb please lol hearts tacee