Question

"You must be saving up to migrate to america" , authoe suggested.
Why did the author make the suggestion ?

Answer 1

When the author asked Nicola what they were doing with the money that they earned by doing all sorts of odd jobs in the city, Nicola replied with a vaguely because his had a mature nature and does not want to disturb anyone with his problems. Now, the author suggested "You must be saving up to migrate to America." in order to get a more clear idea on what the boys intended to do with the money. In those times, it was common thing to migrate to America in search of jobs, hence that suggestion.
Hope you understood! All the best! :)