How can I ask a guy out? ?

ok, he's actually my manager but I'll be leaving this moth in the next three weeks, i'm having a farewell party and invited him as well. how do I convince him to come? if he doesn't come how can I ask him out? its so hard in my situation. if he does come what kind of conversations shall I have with him. it's so nervewracking i've never done this before. thnks.