If he is committed to being a missionary, I don't think it will hurt to tell him. Whether he likes you or not, it sounds like you want to tell him your feelings. Telling him doesn't mean you have to make him pick between you and his mission; just tell him you know that he's going but you can't help it that you like him, and you just wanted him to know.

I think telling him will help you move on. Obviously, you don't want to stop him from doing his life's work, but you can't help liking him, either. So telling him that, and acknowledging the impossibility of your situation, will probably help.

As for when to tell him, if the two of you have any private time together that would be best-- and I would tell him in person if at all possible. But if not, try writing a letter. Don't use the phone; there are too many ways to get interrupted when you're on the phone with someone, and you want to have the time to talk in detail. The advantage of the letter is he can read it whenever he gets the chance, and reply when he is ready.

Of course, telling him won't mean your feelings will pass right away. In fact, if it turns out he likes you, too, there's a good chance they'll get stronger for awhile. But having the conversation with him will help you understand your feelings more, and after that you at least won't agonize as much about what he thinks, since you'll know. Then you can start to "move on." Of course, moving on doesn't mean you have to stop caring about him; it just means that you realize that you can't have a relationship with him, but you can still enjoy his company.