He'd pay taxes as if he sold at fair market value. This scenario has occurred time and time again when someone acquires a lot of wealth through investment. His

He'd pay taxes as if he sold at fair market value. This scenario has occurred time and time again when someone acquires a lot of wealth through investment. His best option would probably be to create an irrevocable trust which pays him a percentage every quarter. Then he won't owe any taxes on what has already accrued. Then he can consider expatriation or moving to Puerto Rico.