Do we need to worry about "drug vacations" and weening him from his Concerta before adulthood?

My ADHD son has been on Concerta for several years. He has had wonderful results with the medication. We recently changed doctors and the new doctor refused to perscribe the Concerta at the level he was taking. His grades suffered and he reported just "not being able to concentrate." He finally got back on the higher dosage, but the nurse practioner that he sometimes sees is still having trouble with the 63mg dosage he needs. She gave him a script for 54mg over the summer because she didn't see any need for him to concentrate since he is out of school. She keeps insisting that he needs "drug vacation days," and actually would perfer that we take him off the Concerta completely over the summer. I have read that taking someone off stimulent medication "cold turkey" is dangerous. She never sees him off his medication, since his appointments are after school when he's been on his medicine all day long. She wants to add a non-stimulent medication like Strattara to the lower dose of Concerta. His behavior is so much better when he is on his meds that I hate to mess with success! He doesn't run ninety to nothing at the mouth, he can listen to instruction and actually do what he is asked, he doesn't do mouth noises and finger tapping, he makes much better decisions and simply likes who he is on his medication much better than who he is off of it. She is concerned that he won't be on stimulent meds his entire life and needs to ween himself off before he goes on to college. He is 15 and weighs approximately 140 lbs at 5'9" --- he is very healthy, happy and well adjusted. We have an appointment with a psychiatrist to evaluate him and officially recommend his medications to the general practitioner. --- Do we need to worry about "drug vacations" and weening him from his medication before adulthood?