Chinese Visa to meet our son in shenzhen

My son is working in Shenzhen China.We want to visit him.The Application Center asking for our relation proof with son by a birth certificate.My son was born before 33 years and we do not have his birth certificate.How to solve this issue.Last year we visited him and no such requirement was raised.We have confirmed Return Tickets which confirms that it is a normal family visit for a month or so.