My teenager fasts but without enthusiasm, treating it as mere routine. How do I nurture its meaning?

Meaningful fasting for a teenager comes from understanding its purpose, not from pressure. At this age, a child can already understand fasting as training in self-restraint and as feeling the hardship of the hungry, not just holding off hunger until Maghrib. Discuss that wisdom, and involve him in acts of worship that feel meaningful, such as choosing who receives charity, preparing iftar food for neighbours, or praying qiam together on a few nights. Avoid comparing him with a more pious cousin, because that kills enthusiasm. Make sahur and iftar family moments to look forward to, full of jokes and warmth. Celebrate his effort, and let him see his own parents enjoying Ramadan, not just enduring it. Further guidance in educating the family.