What should I say to him, I dont want him to go but I feel bad if he doesn't?

Well my boyfriend is a junior in High School. We broke up for about a week and then got back together, and I think our relationship as a couple is 10X better than it was before. We are alot closer. But about a week or so after we got back together we went out to go see a soccer game, and it was tons of fun. We held hands pretty much all day-even in the car on the ride home. But that night he asked me if he could go to prom with a girl that asked him, it is Senior prom by the way, I said it was fine. I think the girl asked him around the time we broke up, or her friend, the one going with him, had a friend ask him to go with her. Either way it really bothers me now considering the prom is tonight (June 10th) He is already gone and I can't help but, I know I am pretty whimpy and babyish, but cry a little over it. I am a freshman, he is a junior, and the girl is a senior. I want to tell him how I feel about it but I am afraid that he will find me 'brooding' or controlling about something that already said and done. I also don't want him to feel bad about it. If anyone can tell me, even give me some advice on this matter I would love it. This is my first relationship so I am not sure what to do, considering I have no experince on the matter of dating. I feel like I can trust him, but I am still worried he will cheat on me with the girl he is going to seinor prom with.