He sucked plenty last year before working the slider into his repertoire.
I won't pretend to know much about pitch usage and run values and that kind of stuff, but it looks like all of his pitches (except the slider, which is new this year) have ranged between excellent and awful over the last few years. Considering how much he struggles with control, I assume the best bet would be to make him a two-pitch pitcher and get him to work on harnessing those instead of the varied repertoire that hasn't really gotten him anywhere so far.