my child's I485 is regected, please help

Hi it said that he didn't sigh the form , so it is injected, but he is not 18 yet, should he sign? Anyway, the rejection noticed said that we need to sign properly and resummit. Actually my child's file needs more documents although they didn't mention it. Should I get the proper documents ( it might take several months) and then summit or should I summit the application first after we sign it and get the documentation later? Thanks