I phrased it poorly, we can cut him in the offseason AFTER the 2017 season, before the 2018 season.
New Orleans Saints Salary Cap Calculator | Over The Cap
click the 2018 tab, then go down to Fleener's name and click the drop-down for the cut options if you'd like to see for yourself.
But the long and short of it is, we're only stuck with him for this season and next.