We never ever had our kids write the words at all. Never. Not even once that I can remember.
We had them in the visor of the car and every time we got to a stop light or traffic jam we'd pull them out and say "Spell The" "Spell Cat" "Spell Hat". Then we'd move to the next time we stopped and do the same thing. Our girl is the highest speller in her school for her age. Our boy isn't reading yet but we have hopes he'll finally click with it in first grade.
Some kids don't learn by writing, others don't learn by speaking the material. I suggest you go google Learning Styles and do a few questionnaires with him to see if you can pin down how he learns.
I test high is audio and visual learning styles. I find it tedious and time consuming to take notes. If I just sit and listen I learn the material. If I hurriedly write notes I miss most of what is said. So I stopped taking notes and started taking a tape recorder to class. I found that I could put the tape in while traveling to and from school and I made nearly straight "A"s.
By figuring out a child's learning style you can find out how to most effectively help them learn the material.
I worked with one person who learned by writing. She'd get the office memo's and copy them in to her personal notebook. I'd get frustrated with her taking so long getting the office stuff done so she could go and do the list of things she was responsible for. I would tell her can't you just read it???
It was so hard working with her but once I stopped and thought about it I realized I had an opportunity to learn a different learning style and see how her mind processed the information.
By writing the word she was putting it in her mind through her hands. But listening I was learning the material through my ears. Others can learn by looking at the teacher and watching their lips and their writing on the board.
Each of us is different and we all learn differently.
I can remember conversations with people from high school. I can't remember a single time I wrote anything down and studied it.
He doesn't need to write his spelling words. If he's ever given a essay exam he's not going to be able to recall a single thing he's studied. If he's said it out loud and been able to verbally put the word in sentences he is going to be able to form sentences and paragraphs easier too.