I dont really agree that 13 is to young to start exercising. I myself am 30 and i started workout out at 13. but Yes the 13 week program is a bit much. I would go with a Rippetoes starting strength program or a madcow 5x5. both simple basic strength building programs that will build an unequaled strength base he will use for the rest of his life. the 13 week program is a great program, but is geared more for losing weight and is a lot of volume for starting out. just my 2 cents. take it or leave it.
Man O steel.