There are a plethora of sites referring to Chihuahuas and blindness...I'm not sure someone can give too much more light on this situation without a lot more details about what has been happening, time frame, etc.
You might try either web searching for "Chihuahua blindness", or get more details from your son and Reply here.
Your concern has touched me--if you have follow-up questions, or need to clarify anything, please let me know right away! Your "Accept" helps me reach out to others!