He's making me spin around in circles.

(Does a Gaydar exist? Could I please have it?) My crush, whom I've been crushing on for the past 5/6 months, has completely spun me around from thinking that he's straight to gay to straight again. I use to never even consider him a possibility... He's a senior on my swim team who just turned 18... I'm turning 16 in October, but by then he'll be gone. He's made me feel wanted and I feel like he cares about my wellbeing. One day when I was experiencing severe stomach pain, he seemed genuinely concerned, like a side of him that I've never seen of him before. And then this past week, we've been joking about an accident that I made to where I fell into the pool with my school uniform on. It just felt so good to finally hear him laugh from something I did. And then there'll be the times where he seems completely and totally straight. He'll joke around with his buddies and make gay jokes in the locker-room and pull the occasional prank. It could just be a thing that guys do, (for some reason I'm just not that type of person). He'll be gone soon, but literally this is tearing me apart right now. I've told myself to hold on on telling him my feelings because I don't want his time remaining to be tarred by my selfishness, nor do I want him to go off to college in a bad mood. I've prayed for strength, but I'm not sure how much longer I can go seeing his smile and hearing his warm voice without buckling and giving my secrets up... What should I do?