There is no real answer as where to draw the line in stopping your child from perseverating on a topic and allowing him the freedom to think and be inquisitive.
You certainly want to encourage your child to learn, ask questions, and become excited about different topics. However, you probably don’t want to talk about airplane engines for three hours, and neither does the rest of the world. There are also many instances where perseverative thinking can interfere and distract him and he will need you to help interrupt the thought pattern.
In the classroom, his teacher can stop perseverative questioning by telling him, “One more question and then we will get back to the lesson.” Parents, relatives, and other adults can use the same technique. If your child persists beyond one more question, he can be reminded, “We are not talking about clocks anymore; now we are talking about what movie we will see tonight.” Or he can be told when it is okay to revisit the topic of clocks: “We are not talking about clocks anymore, but tonight at bedtime we can talk about them again.”