Did your nephew attend Taft or Loomis?
It’s very rare because all three schools ( HCD ) rejected him . It’s also rare because most kids who do transfer usually don’t do it for the academic reason you described . It’s usually the other way around… with kids switching BSs because they’re in over their head. Kids also switch BSs because their unhappy, too … but those kids usually return home.
That said, it’s possible I guess , but I wouldn’t encourage anyone ( who didn’t get into their first choice(s) M10 ) to look at this like it’s some kind of admissions strategy. Same for WL kids ( who have a slightly better shot ) … just throwing it out there! 
Aside from that - The first term III Form year ( arguably the easiest term of all academically ) isn’t in my mind enough time to give any school a proper shot- especially great- rigorous schools like Loomis or Taft. If this happened to me- I’d probably tell my kid to enjoy the slower pace while they can … and focus on getting settled in.
Most boarding schools appreciate all the adjustments, life changes and challenges that kids go through during their first term away from home and start off slow before they really ramp up for a reason. 
Anyway…
@MtnTrailX - I’m glad everything ultimately worked out well for your nephew and hope he’s doing well!
All the best and have a great weekend.