They don't sign LOI anymore because of the way the NCAA counts initial scholarships. If he signed a LOI and was eligible to attend then he'd be counted as an initial. This is a change from how the NCAA used to count them (not until they showed up to campus). Some missionaries sign financial aid agreements, but unlike LOIs, they're not binding on the player (although they are binding on the school).

So if he's going on a mission now, he won't sign a LOI in December (or February, for that matter). If he's not then he might.