I think the key to his success is developing the changeup - a plus changeup adds 2-3 MPH to perceived velocity b/c the hitter has it in the back of his mind, causing that 1/2 second hesitation. And a changeup with a good tail away from RH hitters is a big plus for a LH pitcher.
The cutter is also a key pitch, when you have subpar velocity, movement and subtly changing speeds keeps hitters from keying off your FB.
Seems he has top command, so if he keeps developing the change and cutter, he has a shot at a back of the rotation spot.
Smyly is a good example of a similar LH who built a career as a back of the rotation guy, Smyly added a MPH or two in his late 20s as he physically matured, but is basically a 3 pitch guy, FB, Curve, Cutter.