Answer:
I hope this helps :)
Step-by-step explanation:
A doctor could do multiple things. 
 - The dctor could first prescribe the teen something to increase the Vitamin D deficiency. 
 - The doctor could also try to find out why the teen is not eating and try to fix it.
 - The doctor could also have continuous check up's to make sure the teen is eating. 
 - The doctor could also have the teen go to a pyshiologst, so the teen can talk out his/her problems on why the teen is not eating and how they can fix it.