How do you tell a Mormon missionary that you are in love with him before he leaves?
Take it from a guy who knows and was on a mission once. He doesn't want to know what girls like him right before he goes home. And missionaries ARE different than other regular people. Missionaries are suppose to stay focused on work for their church right to the last minute, not thinking about a girl. Wait until he is home or you may get a cold response (the standard missionary response). If you write, call each other and end up liking each other, long distance relationships do work out - take it from someone who also knows.
God gave us love and to put a muzzle on telling someone you love them is simply not right. Sometimes when things get tough for us it's wonderful to know love can keep a person going. During all wars, many of the men (and sometimes women) kept going when they felt so low because they wanted to fight and survive for the one they loved. Missionary work is about love and by not telling this young man how you feel then it doesn't make any sense at all.
Mormons are no different than anyone else. Because of your religious beliefs it's all the more reason you are honest and speak from the heart to this person. NEVER live with "what ifs". Even if this person doesn't feel the same about you NEVER fear rejection, but be proud of yourself for being honest and giving it a chance. Too often us humans suppress our feelings and regret never telling someone we care about how we feel.
Of course it is important to express our feelings, and to offer love to others. Just, because of the rules of the mission, you don't want the guy to get in trouble either. ... so, wait, and then tell him after he moves to another area. If he likes you, he will write to you, and there are a lot of missionaries who eventually do marry people they met during their missions. Just remember... one kiss and he could get kicked off the mission. That is why it is much, much nicer to the missionary to tell him after he leaves the area.