It is better to play B and get a ton of reps vs being a new kid getting 10-12 reps a game on A team. Additionally the B Team can build confidence. I know this does not make him feel better.
A lot changes in HS. I know B players who became very good HS players and A players who fizzled out in HS. Lots of factors: puberty, non-biased coaching, time for development etc...