I think that getting him evaluated by a psychiatrist is a great idea. Hopefully, the earlier doctor gave you information on how to help him besides just the meds. All in all, information on how to work with a child with ADHD is as important or more so then the meds. Just in case you didn't get much info - I recommend you buy the book, "The ADD/ ADHD Answer book," by Susan Ashley. It will help a lot.
As to your question, if he is doing ok on the meds now, he should be fine. Most potential problems are caused by children not on meds or not getting the proper help. If the med didn't make him hyper (as it would a non ADHD person), then its doing what it should be doing. However, if he is kind of drugged out, or in a stuper, then the med is too high and needs to be reduced or stopped. Seems like your new doc is being pretty honest. Hopefully, she is also giving you resources to help you/him. If not, I can recommend more. But the book is a great starting place. Best wishes.