No, you cannot

He can ask his parents to sign for it.  Just send the document to China and sign it and send back. 

You are not legal guardian in the strict sense.  You are merely an authorized caretaker, in the sense like teachers when parents drop their kids off at school.  Even though you might be authorized (through whatever paper his parents signed) to make medical desicions, that is not enough. 

If a teacher cannot sign for this documnt, neither can you.