According to him, it has a special design with a huge window in the front of the highest deck that will protect the passengers from monsters. Yes, monsters. :)
He finally drew a really pretty triple decker bus last week. Will post pictures soon once I upload them.
Edited to add: New details which he added today. The triple decker bus can take us across ocean to Sri Lanka, Australia, London and many other places like the Sentosa, Botanic Garden and the garden that his grandparents have. As he draws me yet another triple decker bus (this very moment), he is singing the "wheels on the bus".