depending on how attentive he is, I would let him. Does he pay attention to direction and is able to focus on what he's doing? If so, try teaching him the proper way to use a rotary cutter:
1. always wear a rotary glove (at least until he's an adult :))
2. the cutter must be in the CLOSED position except for when you are actively cutting. Investing in the self-closing cutter works great for this one.
3. Teach him hand placement on the cutter (the angle/pressure to use) and the hand placement on the ruler.
4. you may want to invest in the plastic finger guard that can go on rulers.
5. At his age he must be supervised at all times while using the rotary cutter.
6. if he fails to follow any of these rules, he loses the privilege to use the rotary cutter.