Getting a kid from 1500 - 1700?

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ndb2010

The journey for our 2nd grade son from USCF 1300 to 1500 the past school year was largely about tactics, more careful calculations, and slightly better Rooks+Pawns endgames. Now we need to set his study plan for the summer, and wondering which Ipad-based trainings you would devote your time to (he has a lot of time in cars/ planes / buses planned):

1. More tactics (we like CT Art 4.0, though I am told we could just use chess.com)

2. More endgames - ChessKing endgame trainer (or other?)

3. Middle game strategy trainer (Chess.com or ChessKing)

For those who have recently made this journey, which area will give the most return per hour invested?  

Thanks!

MitSud
Coaching and probably middle game plans + tactics
MitSud
The pawn structure series by IM Daniel Rensch on here is pretty good, helps with understanding which is key.