My husband has 40 years old. But is an illiterate person. He attempted learn to read and write, but he did not achieved his goal. He think, that there is a problem in his mind that difficult him to learn. He dreams to be an american citizen. He want to know how obtain this privilege. Thank you for your answer.

Is he a permanent resident now? The naturalization requires basic literacy. You should consult with an experienced immigration attorney to research the "disability" exception as regards to literacy.