He always sticks rigidly to rules and never wants to try new methods.
Tips
history
墨 refers to Mozi (墨子), the founder of Mohism, who was known for his strict adherence to defensive tactics (守城, defending a city). 守成规 = to hold onto established rules. Combined: stubbornly defending old conventions.
memory
Imagine someone clutching an ink brush (墨), standing guard (守) over a dusty old rulebook (成规) and refusing to let anyone change it.