Nature changes stats period, you dont have to use common candies to reset stats, his special attack should automatically be higher if hes modest even if you dont change levels at all. If you dont believe me you can test it out.
I do think for me calm or bold is the best way to go since his typing and defense stats makes him bulky and his special attack is naturally not very high. For me at least i like to use him as a kind of a tank that has stabs that cover alot of my team's weaknesses and i can just revive and heal the rest of my team while he tanks.