My 6 yr old has the same problem. I don't think OP is necessary. I homeschool so have been able to help him a little more, but I just help him develop his fine motor skills by giving him activities that strenghthen those muscles. I bought some clay that was fairly hard to work with and I use dry beans to mix into the middle of it and hand it to my son and he has to treasure hunt to find the beans. I also use the same clay and squash it down in a cookie sheet and have my son write shapes and letters, or draw pictures in it. It has helped a lot, and is a lot cheaper than OP.
Sorry, I just finished reading all the posts, I had just read the first few before. I'm sorry about what your family is going through. Chemo can be tough on everyone. I hope everything turns out for the best. (lots of hugs and prayers)