Tell him this: "Those people who wear those costumes should not be trusted. If you ever see anyone in a costume like that run in the other direction because they are bad and promise me you will never wear one of those because it is really bad"
Introduce him (if he doesn't already know) the universe of FNAF. Although William is a furry, he already likes being a golden rabbit.... HELP SOMEONE HELP MEEEEEEEE
Not really. Tell him the truth. He just does not know. Tell him without taking sides. If he chooses to follow their ways, ask him for a reason. This can be used for preventative interaction.