Assuming there was no trauma to his right eye, then there are very few things, if any, that could be causing blindness in the right eye. Rarely, facial/orbital trauma can cause trauma to blood vessels in an area called the cavernous sinus, which has blood vessel connections to the both eyes. This may cause a carotid-cavernous fistula (CCF for short). In this case, however, there are usually at least some signs to suggest something pathological happening in the non-traumatized eye; however, these signs can be subtle and a CT scan may not pick up this type of problem. This seems less likely however.
I would say that it is not normal that your brother is unable to sign his name when the paper is put directly in front of him and does not coincide with vision loss in his right eye.
Other than that I do not have any possible explanations for vision loss in his right eye. A consultation with a neuro-ophthalmologist might be the best thing for your brother.
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Disclaimer: My opinion is solely informative and does not constitute an informed medical opinion or recommendation. For an informed medical opinion and/or recommendation you must see an eye doctor.