I'm looking for a word that's somewhat similar to inflexible, but non-derogatory, used to describe a characteristic of a person or their habits.
For example, let's say we have someone who really cares about their oral health. He brushes his teeth on time shortly after every meal and tries to keep his teeth-brushing to no more than three times daily because he believes brushing too much is harmful to the teeth. As a result, he tries to avoid snacking whenever possible. So, he doesn't take any snack beverage or any food like chocolate, chips, coffee, fries, etc. outside of his three main meals during his commute or at social gatherings, parties, etc. even when offered. When offered he will politely refuse or save them for later until a main meal.
Now, this is not necessarily a bad thing. His behaviour shows that he is a pretty [...] person, or he is being pretty [...] about his oral health?