I would cut it for him with him observing and being trained in safety issues. I honestly think scissors would be a great teaching tool, however, could end up with wonky cuts.
I started sewing when I was 8 and had to cut my own pattens with scissors (for one reason no one had even heard or dreamed of rotary cutters, what is another story!)
I actually learned to sew on a Singer FW (and wish I had that machine today).