It's not enough that he was singlehandedly responsible for making the first one a reality (he apparently leaked the pre-vis video that went viral everywhere by accident), he also apparently helped craft the story for the first one while shooting Green Lantern.
While Reynolds didn't get a writing credit for the first one, it's now confirmed that he's getting one the second time around. If you check the official Fox website for Deadpool 2, Reynolds is listed as a credited screenwriter alongside Rhett Rheese and Paul Wernick.
Reynolds was already listed as a producer on the first one, and the same with the upcoming X-Force spinoff, but a credit as screenwriter is usually on reserved for people who've actually had an input. In some cases, if there's significant work done by a writer on a script, they'll sometimes get a producer's credit.
So, what did he contribute? Who knows. We'll have to wait until May 15th.