Why? Even if the Green Party implements global taxation based on citizenship, just like the U.S., he may have other options. For example, U.S. citizens can reduce their tax rate from 40% (or even over 50%, if they live in a very high tax area such as NYC) down to 15%, or even less, by moving from the U.S. and creating an offshore business.
Many hedge fund managers in NYC moved to Puerto Rico for the 4% tax rate. Renouncing your citizenship is a drastic remedy that is hardly ever︀ warranted.