I suspect his success so far is largely due to his being an unknown in the NL, getting some easy Ks against pitchers, and that he's been a streaky pitcher in the past and he's on the good side of one of his streaks.
The no DH factor is probably a huge part of it. Besides the fact that you've got an almost sure out in the 9th spot, it also allows you to pitch the 8th slot hitter differently. You can be more aggressive, or if you get into a hitter's count, you can pitch around them to get to the pitcher.
There were periods of time in Boston that he looked this good, then he'd lose it for months at a time.
There is a chance though that he's maturing as a pitcher, but I suspect he;ll come back down to earth, either through the NL adjusting to him and/or he cools off on his own.