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It was an offer letter, not an application; and was sent to me on a Friday, and needed to be returned the same day (and I didn't have access to a printer).

It doesn't sound like he was filling out an application. It sounds like he had the job and was just filling out HR paperwork for benefits and taxes and such.

Potential law suit for disability discrimination?

Or a field which requires fine motor skills.

Yes, that's a good point. Some jobs have intrinsic requirements for which no reasonable adjustment can be made, and thus legal discrimination happens.