If your father is not disable, he needs 15 years of residency in US after getting green card to be eligible to apply for citizenship - this applies if he does not know english.

Exceptions & Accommodations

There are exceptions and modifications to the naturalization requirements that are available to those who qualify. USCIS also provides accommodations for individuals with disabilities.

For more information visit our A Guide to Naturalization page and the USCIS Policy Manual Citizenship and Naturalization Guidance.

English Language Exemptions
You Are Exempt From The English Language Requirement, But Are Still Required To Take The Civics Test If You Are:

Age 50 or older at the time of filing for naturalization and have lived as a permanent resident (green card holder) in the United States for 20 years (commonly referred to as the “50/20” exception).
OR
Age 55 or older at the time of filing for naturalization and have lived as a permanent resident in the United States for 15 years (commonly referred to as the “55/15” exception).
Note:

Even if you qualify for the “50/20” or “55/15” English language exceptions listed above, you must still take the civics test.
You will be permitted to take the civics test in your native language.
If you take the test in your native language, you must bring an interpreter with you to your interview.
Your interpreter must be fluent in both English and your native language.
If you are age 65 or older and have been a permanent resident for at least 20 years at the time of filing for naturalization, you will be given special consideration regarding the civics requirement.
For more information, see the USCIS Policy Manual Citizenship and Naturalization Guidance.

Medical Disability Exceptions to English and Civics
You may be eligible for an exception to the English and civics naturalization requirements if you are unable to comply with these requirements because of a physical or developmental disability or a mental impairment.

To request this exception, submit Form N-648, Medical Certification for Disability Exceptions. This form must be completed by a licensed medical or osteopathic doctor, or licensed clinical psychologist.

For more information, see the USCIS Policy Manual Citizenship and Naturalization Guidance.

Continuous Residence Exceptions
If you are engaged in certain kinds of overseas employment you may be eligible for an exception to the continuous residence requirement. For more information visit our Continuous Residence and Physical Presence Requirements for Naturalization page and the USCIS Policy Manual Citizenship and Naturalization Guidance.
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Disability Accommodations
Under Section 504 of the Rehabilitation Act of 1973, we provide accommodations or modifications for applicants with physical or mental impairments that make it difficult for them to complete the naturalization process. Applicants are encouraged to list their needs in the space provided on Form N-400, Application for Naturalization.

For more information, see the USCIS Policy Manual Citizenship and Naturalization Guidance.

Hard to believe how people are full of bs here. Check the information first hand rather than be.
https://www.uscis.gov/citizenship/educators/naturalization-information