I'd recommend taking some video of your son throwing the cutter. Some of the clips of Rivera I've seen show him turning the wrist a bit like a slider and doing the same thing could be tough on an adolescent arm. It's pretty easy to go from pressure on the finger to turning the wrist without realizing it. As long as he isn't turning the wrist to get the motion it is a great pitch to have in the arsenal to jam lefties and get righties to try to pull grounders off outside pitches.
If CASon were healthy we'd probably have him trying the grip, but he's on the shelf for at least two more weeks before beginning a very gradual rehab.