It depends on his hb1ac really and personal preference after that. Many in here have taken metformin for a period of time, varying from days to years, and then stopped without detriment. If you don’t need it because you have achieved control through diet, exercise, weight loss and maintenance then why take it??
It’s an outdated view that says its automatically forever. That is when they thought nothing could be done to turn type 2 around and achieve remission. They were wrong.