ROFL
Actually Rich's suggestion is a lot more likely to work
Speak to his Mum first to let her know what you're going to do - along the lines of "you know how you said your son is bored and cooped up...." and then get him looking under the bonnet and doing some supervised tinkering - if his Mum's happy (and he's tall enough) take him out for a spin.
Above all - talk to him and get to know him
On the other hand if they're one of these ASBO families who just don't give a sh*t - start recording everything that goes down and get CCTV trained on the car