Have you noticed that a lot of students with autism hold their pencil with 4 fingers wrapped up the barrel of the pencil, and their thumb is pressed straight into the pencil. This grasp is not just specific to students with Autism, as the kids who use it generally have weak fingers.
The reason that these students with weak fingers and limited fine motor coordination use this grasp is that it strongly stabilizes the pencil in the fingers. The fingers wrapped up the barrel act as a rest for the whole pencil, and the thumb pushes the pencil into the finger rest to hold the pencil super stable. Some typical kids use this grasp as well if they happened to like how it stabilized their pencil when they were first learning how to write.
One reason that this is not the best grasp for a student to use on his pencil is the same reason that they do use it. This grasp stabilizes the pencil very very well, so you do not have as fluid finger movements as when a typical tripod pencil grasp is used. With students who have weakness and poor fine motor control may not be able to control the pencil with a typical grasp so they fall back to this grasp, which over time becomes the most functional for them.
As a therapist, you are asking what you can do for this grasp. Once this grasp is established by first and second grade, it is almost impossible to change it. Before it is established as a strong habit, there are some things that you can work on to try and get a more fluid and functional grasp. The first thing that is needed is a thicker pencil or pencil grip. Since we are dealing with finger weakness, we need to make it easier to hold the pencil in the correct way, or they will be forced to hold it a different way. There area variety of pencil grips that can be tried, and some kids will resist some but be agreeable to others. I use the Ishy grip for extreme weakness, but it is too bulky for the kids with mild weakness, and a different grip would be better.
Along with making the pencil easier to hold onto with a built up pencil grip, some hand and finger exercises are good to do. Specifically, finger activities that work on finger pinch strength, finger movement/excursion forward and back (like threading a needle). The finger exercises need to be done with the thumb and fingers held with a curve (bent at the joint) to strengthen the necessary movements for writing.
Have you found some great ways to work on this problem?