You can try every pencil grip available at Amazon, and you may have no success with changing the way a child holds their pencil. Really, the simpler the solution the better, and if it is simple, it makes it more likely that the child, teacher, and parent will be able to do it every day so that it becomes a long term solution. My favorite, most simple solution to helping a child’s pencil grip is to have the child hold a little ball in the palm of their hand.
I have made little devices to try to make it even simpler, but when it comes right down to it just grabbing a pom pom or little porcupine ball is really so simple. You could use a bouncy ball (not recommended as they bounce all over the room), a piece of wadded up paper, a paper clip, an eraser, or any other small toy or bead. I do like to use the ball in the hand in combination with a jumbo grip to get a really good grip, but it is not always feasible to put a grip on every color of crayon, and they don’t fit on big markers, so the ball works on its own in a pinch.
If you can get the student to use the ball whenever he writes, it will help his muscles get used to working in that position and will strengthen the little muscles of the hand that work on writing. In a while, when the muscles are stronger and they are used to working that way, the ball will no longer be needed. It really does work well.