Let him use both hands to write letters and words, It is a marvelous activity. Just gating the feel of large movements may be sufficient without introducing formal spelling to the activity. Let him make up his own codes for you. You make up one but make sure he has a way to decode it. If your child is working on a class spelling list and can only remember half of the words, speak to his teacher.
Teachers are more than delighted to hear how their students respond to homework.
Perhaps the list can be reduced so that your child has fewer words and can learn these more efficiently and comfortably. Don't tackle an entire spelling bar in one sitting. Take one-third for example, each evening, to work on with your child. Break the practice into small units. Shorter periods given frequently are more effective than one massive review-which is also exhausting and frustrating.