Extending 19 yr old son's L visa

Hello, can anyone please help me! We are long time residents of China and both my husband & I have work visas. Our 19 year old son's current L visa expires a week prior to his leaving for college in the US. We want to extend his visa in China if possible. It seems the only way is with a visiting family visa. Unfortunately, I do not have his original, nor a copy of, his birth certificate. We have tried to confirm what docs are acceptable forms of proof of relationship (other than the birth certs), but no success with the visa bureau. Has anyone out there done this recently and can help me? Thank you!