So I think he needs to attend fall semester. The rule with BYU is that you can defer one semester for any reason and you can defer 2 years for a mission.

You cannot do 2 deferments in a row.

My brother went through this and had to reapply when he got home because he didn't want to go to school before his mission.

To be safe I would just have him take an online section of an on campus class. That is different than an independent study or regular online class. A good one is Organizational Behavior 241(or close to that) by Cherrington. He works with kids all the time in similar situations. I know several people who have taken that class to keep their spot while they were somewhere else in the country