How can someone reclassify while still in AIT?

He cannot reclass. He could fail out (which is horrible recommendation) and then he would go before some type of Academic Deficiency Panel where they would make a determination of recycling him to a different class, reclass, or kicking him out. I would say he needs to figure his life out and talk to his unit's MFLC (Military & Family Life Counselor) or maybe a Chaplain and get some good counseling to help him sort out these emotions and make a determination on if he truly doesn't want this job or if there is something else going on that is a temporary issue that can be addressed to alleviate his frustrations and allow him see that its not as bad as he currently believes it to be.

Through my years of Active Duty, I've either known, observed, or had Chaplains Assistants as my Soldiers. In garrison, they are basically the Executive Assistant and "Chief of Staff" for the Chaplain. That means the Chaplain's Assistant manages the Chaplain's schedule, controls access to the Chaplain, manages religious programs, etc. Chaplain's Assistants also prepare religious services, control religious service accessories, the chapel, etc. It keeps one busy, but is hardly a "sucky" job.

Deployed, Chaplain's Assistants do all the garrison stuff, but are also the Chaplain's bodyguards, as Chaplains are not allowed, by Geneva Convention, to be armed. So 56Ms are "combat" personnel downrange.

I have known several Army Chaplains, and they have all been really great people to know, to be around, and have access to. One in particular for whom I have enormous respect and awe, is a former police SWAT officer, as well as a former Navy Sailor.

Tell your brother that AIT sucks, no matter what the MOS. It will get better once he graduates and gets to his first duty station. He may even come to like his job. If he doesn't, well he can reclass to a MOS that he feels he may like better at the end of his enlistment. There is also a voluntary reclass request, but he has to be in his current MOS for a certain amount of time (1 year seems to come to mind, but it could be different). But that's another whole discussion.

I don't believe you can just reclassify after while in AIT unless you fail AIT and I don't believe it is guaranteed.