My question involves criminal law for the state of: IL

A 16 year old was arrested for having possession of my son's stolen bike. The officer who arrested the kid called me to tell me that they released him because he said he had no knowledge of it being stolen. My son and a few of his friends followed the kid to a Subway where they then questioned him on where he had gotten the bike. The kid proceeded to call the boys fags because my son said he was calling the cops and in the meantime was saying that he stole the bike from another kid who apparently was the one who stole it from the school grounds. What can I do about this and who is the cop to determine on whether or not he's guilty. Shouldn't the cop have put in the paperwork as possession of stolen property and then let his lawyers or judge determine if he's guilty or not?