How is a cut fastball or cutter gripped and delivered?

My 12yo son throws a breaking ball, but it is not a traditional curveball or slider. I was wondering if it was a cutter. He starts with his index finger wrapped along the opposite stitching (example if he were gripping a two seam, he would have fingers on left and right seams, he puts his index on the right seam), and his thumb on hide (no seam), ring finger lightly resting on on hide (no seam) and lets it rest loosely in the web of his finger and thumb. He doesn't twist his hand down into a handshake position, which we have been warned against, he finishes with his index and ring finger on top of the ball, and has a complete motion and follow-through, but the thing has an obscene breaking motion and moves on average 60 to 65mph. I have tried to give as much detail as possible, and I hope it's a description that will help in figuring out what he is throwing.