Struggling with something that's entirely new to me.

Since I've seen similar problems to mine posted here and I have no gay friends to talk to, I humbly ask for your help. So I'm a guy on my final year of high school and there's this other guy that I've been in love with since the first year. I have been friends with him since the start of high school (I had fallen in love with him shortly after we became friends). He has always been gentle and very kind to me. I guess what I love the most about him is that he's so lively, nice, sociable, helps me when I need without expecting anything in return and always tells a good joke that manages to cheer me up when I'm down. I guess you could say we're somewhat polar opposites (since I have a lot of troubles socializing with other people and tend to mantain a certain distance from others (of course we also have many things in common)). I've been dealing with this anxiety of telling him how I feel for him these past three years. But I am so afraid as I have never dealt with this issue in my life (It's actually the first time I felt something like this about a person). I'm not terrified of rejection (well, maybe a little bit) but I'm purely afraid that if I tell him we'll drift apart and lose this friendship we have.... The other problem is that our time together is possibly running out. After June it's possible I never see him again and that I'll regret not telling him. Also, statistically speaking it's more likely he's straight than gay (I'm basing myself on global % of people who are heterosexual vs % of those who are homosexual) and therefore, there's a greater chance of rejection. Those of you who have had this experience, what advice can you give me?P.S. He also doesn't know I'm gay (well, at least that's what I think).