The scariest part could be if Timmy discovers a dark secret about his family. Maybe he finds out that his ancestors were involved in some sort of dark magic or cursed activities. This would bring a sense of dread as he realizes that this dark past might be affecting his present life in some way.
Perhaps the scariest part is when Timmy gets lost in a haunted forest. He hears strange noises, like whispers that seem to be coming from the trees. The shadows move in a way that defies logic, and he feels as if something is watching him from every direction. Every step he takes could lead him closer to an unknown and terrifying danger.
Well, it could be that Timmy has a doppelganger in his story. One day, he starts seeing this exact copy of himself, but the doppelganger has an evil glint in its eye. It begins to mimic Timmy's actions but with a malicious intent. Timmy can't figure out how to get rid of it, and it starts to take over his life, making his friends and family turn against him.