It's not a belief he's planning to discourage too quickly; there's no harm and a great deal of good in it.

"Then what should you do?"

The prompting is gentle, but he is going to follow through - Legolas could be flighty, and make him slow down and figure things out himself, then say what it was, was one of the few ways Thranduil had found to keep him from forgetting the lesson instantly, especially without Thranduil personally becoming stern.