With respect to the whole power aspect, legally airsoft guns have to be below around 350 feet per second (ish) if they can fire in auto. Any gun you buy online or in store will have to comply with this, BUT if you feel that this is too powerful for your 14 year old then you will need to ask to have it downgraded before you take delivery. Thing is that airsoft inherently comes with an amount of risk and indeed pain if you get shot. If you're looking for something more akin to the £40 Cyma he had before then you'll potentially be out of luck. That said, there's no recoil and he can always get protection/padding if it's a problem.