I'm sorry that your father is having trouble communicating through writing. It must be really frustrating for him and your family. I am not sure whether any of the writing products we sell in our shop would make his writing more legible because they are designed to make writing more comfortable for people with Parkinson's. but you are very welcome to have a look and see if they could be of any help.
We have a writing grip to make standard pens more comfortable to write with and a light touch pen with an ergonomic design for those who have lost dexterity and find writing tiring. If you think either of these products might help your father's writing you can order them by phone or mail, but please do email firstname.lastname@example.org before ordering to ask for a delivery charge quote for shipping it to India.
I hope you find a product that can help your father communicate more easily and effectively.