Stress that he was in the house when he had the knife, and they removed him from the house. This may sound like a silly question, but does he always carry the knife in the same jean pocket so as there is a well worn mark indicateing the knife is always carried? The officers may not be that inciteful, but I have seen that type of fact brought into court when a defendant stated he did not know how the knife got in his pocket or the the police planted it there. I am pretty sure in FL the charge will be dropped, but get you facts lined up in your favor. Good luck!