Page 26-27 of the <a target=_blank class=ftalternatingbarlinklarge href="http://uscis.gov/graphics/services/natz/English.pdf">Naturalization guide book
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Exemptions to the English or civics requirements:
1) Applicants over 50 and have lived in US as a Permanent Resident for periods totaling at least 20 years do not have to take the English test, but can take the civics test in the language of their choice.
2) Applicants over 55 and have lived in the US as a Permanent Resident for periods totaling at least 15 do not have to take the English test, but must take the civics test in the language of their choice.
3) Applicants over 65 and have lived in the US as a Permanent Resident for periods totaling at least 20 years do not have to take the English test, but can take a simpler version of the civics test in the language of their choice.
The first 3, you can just write a letter to include with the form (edit: to clarify, send letter with N-400 when he is first applying), or send one to the district office after receiving your interview appointment notice. You'll also want to bring an interpreter to the interview.
4) Exemption based on a physical or developmental disability. Must apply for
Form N-648 with the N-400 application.