I’d say Dev is using a rather negative approach, but this is a very personal question, based on how the reader themself views health too... However i say negative because Dev is comparing himself to a friend. His motivation to be healthy is there, but it is dependent on his Bretts lifestyle and how he looks and feels.
Brett is healthy because he is active (goes to the gym), sober (doesn’t drink alcohol), and perseverant (he has no sick days in school)
Devs health would benefit, as his weight would decrease considering he hasn’t done any physical exercise in 5 years, his feeling of being unfit and unhealthy would decline, most likely contributing to his mental health as well, giving him a better sense of security and happiness.
