Why people do this is beyond me. It would have been so much simpler if they had said that on his 5th anniversary, subject to whatever conditions they choose, he gets a BONUS of $250,000. By making it "$50,000 per year" makes it seem more like a "plan" and it is actually meaningless because it's "all or nothing" after 5 years - the only number that is relevant is $250k. I would argue it isn't a plan - it is an "employment agreement" inartfully drafted.
Tell them not to do it again.