As Buddy said, put a request IN WRITING stating that you want him to "be evaluated for possible special education services including but not limited to academic, psychological, behavioral, speech-language, and occupational therapy evaluations". Send it CERTIFIED MAIL with RETURN RECEIPT REQUESTED to the Superintendent AND the Director of Special Education Services as soon as you can. If you are in the U.S. and he's attending a public pre-school, there is no reason they can't do the evaluations. As Buddy also said, if they won't, you have rights to appeal and more than likely win.
As for the medications, you definitely need to have that looked in to. Wrong medications and/or wrong dosages CAN make things worse. My son, for instance, cannot take stimulants for his ADHD. They make him VERY aggressive and more hyper.
Definitely find a pediatric neuropsychologist, as MWM said. They are VERY thorough and look at more than just the behavior itself. They analyze it and try to determine WHY. That is what you need to know. The good ones can have waiting lists but ours was worth the wait, and then some.
Keep looking for answers. He's suffering as much as you are if not more. You should read the book The Explosive Child by Dr. Ross Greene as well as What Your Explosive Child Is Trying To Tell You by Dr. Douglas Riley. Both offer a LOT of insight into our kids' behaviors and how to effectively deal with it. Hang in there. We'll get you through this if you let us.