Depends on why he travels (e.g., was he a visitor or is he still a resident of Canada by virtue of those visits) and whether he established himself as a resident of the US.
He needs to have established his home abroad and they can ask for proof (e.g., his US work permit). The
instructions state:
"Who are considered Former Residents?
Former Residents of Canada are persons who are returning to Canada to resume residence in Canada after having been residents of another country for a period of not less than one year. Persons who establish themselves as residents of another country for a period of at least one year may make return visits to Canada (as non-resident visitors) without jeopardizing their former resident entitlement.
However, persons who do not establish themselves as residents of another country during their absence from Canada, such as those on extended vacations, voyages or world cruises, are only eligible to the former resident entitlement if the duration of their absence is a continuous period of at least one year, without any return to Canada having been made during that time. Should these persons make return visits to Canada, they will be entitled to a personal exemption as described in the publication called “I Declare”."
. He just needs to turn in the B4 once he has the intention of moving back for at least a year.
