All he has to do is renounce his US citizenship. There is a process. But he probably comes across the border a few times and doesn’t want to jeopardize that in any way.
Even if you “renounce,” you’re still on the hook for maybe ten years’ worth of income.
He does have to renounce his Americn citizen ship. Born of Canadian parents, he can claim Canadian citizenship through his parents and have dual citizenship. So says a Canadian i8migration lawyer wit whom I spoke.
A friend of mine did that. Very expensive process, applying alone is a few thousand bucks w/ no guarantees.