He should be happy if he gets to 2000 by the age of 20.
Unless he has a very low aptitude for the game, he should cross 2000 much sooner.
That was not my statement. Check what Carlsen has to say when someone asks him the same question.
@nimzomalaysian
He was making a little joke about rating the same as age.
@ChessofPlayer
I wouldn't call beginner at 13 to 2000 at age 20 very low aptitude. If you look at chess players in general that's probably closer to average. Yes, some are 2500 by then, but these players are less than 1%
When he wins a game, give him a carrot to eat.
When he loses a game, give him a stick to eat.
Stick and carrot, the diet version. Not only making a grandmaster, but also a very healthy diet.