Well let's start for what the diagnosis is. You might think its bipolar but a psychiatrist could tell more. If you are concerned and think your son has been wrongly diagnosed and is over medicated seek a second opinion. Now let's go through the medications. First of all that's a lot of medications to begin with especially for a child and even standard psychiatry has concerns about use of antipsychotics in children due to the risk of tardive dyskinesia and diabetes. But are they for psychosis or for mood stabilization as Lithium, Depakoate and Lamictal are mood stabilizations that don't have the long term effects of antipsychotics. Lamictal from what I have found is the most tolerable and doesn't promote weight gain. Prozac can be used in children but there are some concerns. If your child has depression then they would use that. If he has bipolar then they would give it as an adjunct for depression if the bipolar did not treat the depression as well as it could. Ritalin of course if for adhd and is of standard usage as is Strattera which in not being an amphetamine is somewhat safer.
As for the reasons, some psychiatrists will use polypharmacy that is provide medication for every symptom and here it seems like its gone too far. But you might be wrong the original diagnosis. The best thing to do is see another psychiatrist, preferably a trained psychopharmocologist who is highly knowledgable, gives you an exact diagnosis and prescribes what medication is needed and no more than that. That's probably the best solution.