Vest his 50% equity but instead of using time, use "customers" he bought on board(or define a different way to measure his value) and decide an amount of custom

Vest his 50% equity but instead of using time, use "customers" he bought on board(or define a different way to measure his value) and decide an amount of customers he should bring in order to make it worth those 50%. Give him them the equity that he deserve based on this performance. :)