I'm thinking that something along the lines of the original religious-themed ones sounds good--remember, this isn't just some nameless pilot out in the middle of the war, he's going to be the main character of the strip, and he should have some sort of callsign that sounds like it. With that in mind, I think that "Deacon" sounds bad-ass, but "Reverend" is probably a more likely 'real' nickname; as in, back at the Academy, he was always the one to have the right answer, or he reminded others to "do the right thing" (or something similar), so he might have been considered 'preachy' by some of the smartasses in his class. Also, while "Reverend" retains the authority and respectability, it can also be meant in a more light-hearted teasing manner, which, as has already been stated, is usually how one gets one's callsign anyways.