Parenting Teenagers (12-18) Support Group
This community is focused on the joys, challenges and concerns faced by parents of teenagers (12 to 18 year olds). The major areas of child development include: physical development, perception and sensory development, communication and language development, cognitive development, emotional development and social development.
I'd start with your regular doctor and then go to his regular dentist.
I would really wonder if something is up.
His regular doctor can talk to you about how to deal with the depression also.
Is he drinking alcohol...at 15?
I've never heard of soda staining teeth. But if he's drinking a lot of it he may have nutritional issues that are the problem.
The behavior changes are a result of the teeth problem, so I'd try to find the cause of the staining first (with his doctor or his dentist) and save the depression treatment for if they can't find a cause (but I'd be unlikely to accept a lack of explanation).
Best wishes.