Seems like he should be keyboarding or using speech-to-text software anyway, if the dysgraphia is pretty severe. It sounds to me to be unreasonable, unless he is only writing a few letters or words. I'm not sure that it would do any "harm" other than being very frustrating. Not sure if it would do any "good" or not. How does he feel about it? If he doesn't like it, I would advocate for him to be allowed to type or have someone scribe for him. I doubt there is any research on what happens if you force someone with dysgraphia to write with the non-dominant hand.
