Can a brain damaged person become a US citizen?

I teach English as a second language. One of my students (age 27) is not mentally ill or disabled, but brain damaged. He lives with his immigrant parents (who are US citizens) in an apartment. He received a blow to the head at age 10, and has some huge learning disabilities. Learning English is a huge challenge for him, but he is learning, slowly. He has a factory job, and is hard working and honest, but I doubt he could learn enough to pass the US Citizenship test.

Is there any sort of legal provision for him to become a US citizen, or is there a way to allow him to become a permanent resident? Does anyone know of other options I can suggest to the family?

One obvious answer is to consult an immigration attorney, but I am trying to find out information for the family. No snarky comments, please.