Hello Lynn,
It would be helpful to know how old your grandson is and what type of difficulty he is having. Has this been an ongoing thing or just a recent development?
This is off topic in terms of EFT, however, there is a book called, MATH DOESN"T SUCK by Danica McKellar. It was a top 100 ranked book on Amazon.com for 2007. The book may give some insight into your grandson's issues. Just a thought.
As for EFT, I would use setup phrases such as, "Even tough I hate math because I am not good at it..." or "Even though Math is hard for me and I feel stupid..." or maybe,"I get so nervous and tense when I have to do math, I choose to relax and feel calm and confident."
Without more specific information about your grandson, these are just some general ideas that might get you started.