Although being able to generate a large amount of turn is fantastic (i know because i spin the ball a lot as well)...
Variation is key in any spinners arsenal, i used to have the same problem but being able to turn it a whole load isnt useful unless used in small doses as the batsmen become accustomed to it or will change their batting position to accommodate (many batsmen i've bowled against set up at leg stump).
Varying Pace, Spin and Flight is the key.
For Example:
1st ball: he could bowl a ripping off break like he usually does
2nd ball: Slightly slower with less spin
3rd ball: Toss it up, maybe bowl a toppy
4th ball: fire it in! quick pace, flat spin isn't an issue just fire it at the stumps
5th ball: Bowl another ripping off break but maybe toss it up a little more
6th ball: (Whatever you like ;D)
basically what i'm saying is while he is still young, he should begin to learn to variate throughout his overs so he doesn't become easy to play after the first few overs.
one of my change up overs consists of:
1st Ball: Arm ball (nearly gets em every time hah)
2nd Ball: Big turning off break at a nice pace
3rd Ball: Short flighted Toppy (my stumping ball it works very well, appears to be a full toss, dips catches them out short and goodbye bails)
4th Ball: Underspinner fired at the stumps
5th Ball: Quick Off Break
6th Ball: finish with a ripper.
What Dave suggested (that he learn the arm ball) could be useful, but for a youngster learning the arm ball can be.... you could say difficult. The Arm Ball basically outswings in flight and carries onto the keeper, it's not an easy delivery to master and it depends on conditioning of the ball and the weather.
So i suggest he vary pace, spin and flight
- Mr Off Breaks
PS: If he wants to learn the arm ball and is that gifted hah read my guide or message me for a more thorough explanation