Mp5k, galaxy, buy it new. He will like the box, and it will have a warranty. It will be good looking, light, and durable, and it will come with everything needed to play and have fun as a 13 year old. Accurate enough to practice shooting targets, use for fun, and a little field play. Low enough FPS to use in CQB OR Field, but good enough for both. Low enough fps to prevent him from injuring himself or breaking things by accident.
Cheap enough that it wont be a sore loss if it finally dies on him, but reliable enough to last.
New mags are 3-5$ and MP5 accessories are the cheapest on the market.
any questions? Pm.
He will not regret it.
http://www.amazon.com/Galaxy-Airsoft-G5K-Electric-style/dp/B001RV9IJKGo for the PDW version if you can find it.