I just want to second the Pediatric Neurologist suggestion. My son (now almost 7) had a number of issues all through preschool that seemed "borderline" to his teachers, and finally in fall of 2009 they said: it's time for an eval. We saw Dr. Raymond Tervo at Gillette (St. Paul location) and it was a wonderful experience. The evaluation process took almost nine months and we (and his teachers) filled out endless forms - but it was absolutely worth it. In the end, we all (teachers, his dad and I, and the doctors) felt totally confident in his ADHD diagnosis. They really covered all the basis - extensive psychological evaluation, an OT evaluation, vision, hearing. (As Dr. Tervo pointed out - if he has trouble hearing or seeing, that could cause all sorts of behavioral problems.) We received a ton of support through the process of deciding whether to try medication (including a three-week blind "trial" of a placebo, a low-dose stimulant, and a higher-dose stimulant with objective behavioral evaluations to compare each week.)
We had also held my son back from kindergarten and have been so glad that we did. Even though we had not completed the evaluation process by the time he started in the fall, we had come a long way - and he was ready for kindergarten in so many other ways. We did end up talking to his teacher; although we didn't do an IEP or a 504, we were able to share some ideas about accommodations that we thought would be useful in the classroom, and they were all things she was happy to integrate.
Ultimately, we have been doing OT with him since last summer, and did decide to go on medication in the winter. These two interventions have worked really, really well for him. He is happy and doing really well at school - he loves school, is always excited to be there, and disappointed when there's a vacation day. We continue to see his OT once a week, and have an OT routine that we do after school (most days.)
As several others have pointed out, there are often several issues going on at once, and it can be challenging to parse them all out. That's why I strongly recommend going to a specialist and really taking your time to get the right diagnosis. Also, don't be afraid to challenge possible diagnoses if they don't feel right to you. You know your child better than anyone else. Chances are, when you hit on a diagnosis, it will feel like a huge relief - if it doesn't, keep working until you find the right thing. Same goes for medication, if that turns out to be appropriate. The right med at the right dose makes all the difference.
Are you at the beginning of a long journey? Yes. Is it a harrowing journey? Likely not. Try to look at it as an adventure that you're embarking on with your son. My son actually really enjoyed most of the evaluations, and we worked hard to make sure that he understood we were just trying to figure out how his body worked, so that his "silliness" (as he called it) didn't get in his way. (I have an anxiety disorder for which I take daily medication, and in the end he and I were able to talk about how our different medications help our too-fast brains to slow down just enough that we can do the things we want to do.)
I also guarantee you and your son will feel a whole lot better about life in general when you have tools to address his body's needs and you both no longer feel like he's a "discipline" issue. He won't feel like he's "wrong" all the time anymore, and you won't feel like you have to play bad cop all the time.
Having spent some time "in the system" now, I would also say this about what some people refer to as the "increase" in autism and ADHD diagnoses . . . When we were growing up, some kids were just weird. Or just didn't do well in school. Or got in trouble a lot. Now, we have tools to understand the amazing variation that exists in human bodies, and the ways in which some of that variation can make living in our society/culture quite challenging. We don't need to eliminate this variation - my son is still the same enthusiastic, energetic, physically engaged, silly funny guy that he always was. But now he has the patience to read a whole book, or write a whole story, or listen to a friend while they're playing, or tell a friend what he needs or wants while they're playing. And we recognize that his body has specific needs for certain kinds of stimulation - and we have productive, appropriate ways for him to get what his body needs. He has also learned about what his body needs, and is able to recognize when it needs something, and ask for an appropriate tool to give it what it needs, instead of just flailing around, out of control. Not that he doesn't still lose it sometimes, but don't we all?
I will also say that the fact that he's young (relatively speaking - most diagnoses of ADHD don't come until 8 or 9) says two things. First, his symptoms are likely quite pronounced, as all preschoolers would fit into the ADHD column in some way. Second, you have a unique opportunity to get him on the right track with school from the beginning. Most kids who aren't diagnosed until 8 or 9 struggle through the first few years of school - your son will have the benefit of learning how his body works while he's learning how school works (in kindergarten) and likely not experience school as a struggle, perhaps even experience it as a joy.
I wish you all the best as you and your family start down this path. There are good things waiting for you all along the journey - most of all, discovering your son.