In all fairness hes pretty screwed. The Corps isn't going to change his mos and send him to SOI after they invested the time and money training him as admin. He'll probably have to end up waiting a few years and then can try to swing it as a re-enlistment bonus, or Marsoc is taking all MOSs now. But, I'm not sure if he'd be in admin or would qualify as a trigger puller. Plus, he'd have to wait until hes a Corporal to try out, and that'll take a year or two to happen. His mistake was going to bootcamp with an mos he didn't want. Let this be a lesson to the poolees out there.
Secondary MOSs really only apply to people holding b billets. As in, a drill instructor whos an 0311 has the secondary mos of 8511. Or a supply Sergeant who is on recruiting duty gets 8411 as a secondary. They aren't going to send an admin bubba to SOI only to return to his desk job.