I would second the 50mm Color Skopar 2.5. It is small and inexpensive, but gives good quality. Also, when you think he is ready for a second lens, check out the 90mm f4 Elmar, which can be had in great shape for under a hundred bucks.
Also, just to play the devil's advocate, for the cost of pretty much any lens other than a Jupiter (if you happen to get a bad jupiter copy, and the kid cant get good, repeatable results, he might get turned off from photography) you could pick up a film slr with several lenses, or a brace of fixed lens rangefinders. Film slrs are also a bit more 'teenager proof' than leicas, as they can generally take a bit more rough handling before things go out of adjustment. You also get the added benefit of encouraging the kid by saying 'if you do well with this camera, and you enjoy photography, you can have a Leica CL when you turn (insert age older than fifteen here). CLs can be a bit finnicky, with more plastic than many/most other M cameras, with the added problem of parts availability. IMHO, not a camera to be handing off to a 14 year old, unless you don't care whether or not it works by the time he is old enough to drive. Consider picking up a Pentax K1000. I would wager that more people have learned photography on a K1000 than on any other specific camera in history, and for good reason. It is completely bombproof, easy to fix if it breaks, it has all the features you need and none you don't, it is cheap, and it takes loads of lenses, including non-pentax manufactured K mount lenses such as Ricoh lenses, which are super cheap because they don't work on Pentax DSLRs. I found a 10 dollar K1000 which needed a light CLA, which I was able to perform myself in the shed, I gave it to a friend who was able to get a Ricoh 50mm f2 lens for it for 15 bucks shipped, and it works like the day it was made. 25 bucks, or even if you buy a kit off of craigslist that might as well be mint for 150 bucks, is a lot less than 200-400 for a lens and 300-600 for a body if the worst happens.
