effectively deferred for 3 or 3-1/2 years if he goes on a mission or at this point he has deferred for basically no reason other than potentially going on a mission. Almost all colleges require an accounting for your deferred admission. You have to be in some sort of structured program. Most schools will not just allow you to defer and do "nothing".
This becomes a logistical nightmare (but I agree that it is manageable) to allow large gaps in deferment. Ultimately, if you don't have a mission call in hand then you better be on the phone to BYU to figure out what to do or you better be taking classes.
I think the easiest solution is don't defer until you have a call in hand.