what are the documents to get china visa for 3 months

Invitation letter from your son and copies of his passport, residence permit, work z and residence permit are required for application. If you will need to submit your application to CVASC, you need to make an online appointment at CVASC's website. Basic documents are your passport, application form, photo.

I want to stay with my son and his family for 3 months.Let me know can i get one for 3 months stay.

You can fill your request of 3 months stay to the application form (that is to say, write down 90 days to the term as "Your longest intended stay among all entries of your intended visits in China ), however, the result is up to the consular officers as they make the final decision.
Anyway, even if you get an L with 30 days or 60 days, it can be extended while you are in China.