Have him learn "the old fashion way" first... with scissors and a fine marking pencil. Accurately cutting with scissors will go a long way in learning to cut accurately with a rotary cutter. Safety first is a must.
My dgd started sewing at 5 and she is 12 now and still does not use the rotary cutter.
The other thing is to purchase a clutze glove ... this is a chainmail glove that if the cutter slips and hits his hand, he won't get cut. Investment is about $30.