At the age of 14 I would recommend building a solid foundation and increasing sheer strength through plyometrics and body-weight exercises.
It may be helpful if you relate this analogy to him:
Picture your body as a block of land. If the block of land has shitty foundations (sandy soil), such as;
Poor sleeping habits
Poor core strength
Poor eating habits
Poor technique during excercise
Then no matter how hard you work, the house you build atop it will crumble, crack and sink into the soil.
However if you address these issues at the beginning and throughout a training regimen then the "house" (your boy) will be able to lay the foundations and not have to worry.
The house can then begin to take shape, beginning with the bodyweight (pushups, chinups, pullups, crunches, lunges and 'the plank')
and plyometric exercises which will look something like this ;
Then after a solid foundation has been built, you can continue to build upon it and "complete the house" by beginning free-weight exercises such as the Benchpress, Squats, RDL's, DL's, Military Press, Powercleans i.e. Compound exercises.
The possibilities of exercise are endless. As Olyy above me has said though, it will not be beneficial to him to engage in "beach muscle" activities. These include things like bicep curls, bicep curls, bicep curls and bicep curls. (Essentially, exercises you could picture a young David Hasselhoff doing in a lifeguard patrol hut
Compound exercises (exercises that engage more than muscle group) will be the most beneficial, but it is imperative that they are performed with correct technique. Otherwise, injury WILL occur.
These exercises should be practiced with a very light weight to begin with, (I would say no weight but it is near impossible to achieve correct form with no weight) and an importance should be placed on achieving correct technique. Gradually (I mean over a period of weeks) allow him to increase the weight load, as muscle gains mass very quickly (Due to the muscle having resistance placed on it for the first time and recovering from atrophy) as you begin then gradually becomes harder to build upon. Then you will see improvements in the "house", just as if you were building it.
As for it being dangerous to engage in these activities at a young age, the only issue with beginning weights programs too early in life is to do with human growth plates (affecting the growth of bones). As such, I would not recommend a 9 or 10 year old begin a weight program, as they are no-where near appropriately developed and that would just be silly.
If you feel that your boy needs a leg-up on the competition, it is very important that he doesn't go straight out and begin free weights. I may slightly be flogging the example but this will bring the house tumbling down into the soft soil, reducing it to rubble.
This is where strength begins, helping your body adapt to having resistance (weight) placed upon it and its muscles.
The last thing I will say is that the key to all of this is CONSISTENCY. Muscle groups adapt to weight placed on them, and grow in accordance to the weight they are used to lifting. Muscles have a form of memory, named (you wouldn't have guessed it) muscle memory. This will remember the rough amount of weight it is used to, but just like all of us it tends to forget the exact amount, and gradually begin to fade away.
I hope I have helped some, and if you need any more advice at all feel free to PM me and I will give you my e-mail address!