[ Lesbian, Gay, Bisexual, and Transgendered ] Open Question : There's this guy at my school, he is a senior and is leaving in two weeks. Help?

I am bi and in my sophmore year of school, and about to be a junior. There's this guy that i really fancy and i wanted to get ideas on whether i should pursue on what i wanted to tell him. "that i like him." He's a senior and leaving in two weeks, i have a bit of evidence saying that he may be gay, bi. Here they are 1- He's hasn't had a girlfriend in the total of 2 years that i have known him, and this is weird because they practically throw themselves at him. 2- He's always sweet when he talks to me, always says hey and sometimes i feel like he really wants to say more but it's always pushed out of his mind and he just walks away with a fake smile. 3- He hangs out with a lot of girls, and is never interested in them. 4- Goes to prom and never even dates them. Those are all of the reasons that i have now, but also i wanted to know should i write him a letter saying how i feel but that there's a good chance that he is bi and that i would just like to be friends?