There are other options also, not just BiPolar (BP) or ADD. I do think he needs to be re-assessed. If/when you do take him to someone, I would go to someone who doesn't just specialise in one type of disorder, because doctors tend to see what they're accustomed to seeing. I call it the revolving door diagnosis. I was once involved in a support network for people with immune dysfunction and one doctor on our list suddenly began diagnosing almost very patient as being hypoglycaemic. Every patient would leave his surgery clutching a bottle of glycerin, which they were supposed to sip at frequent intervals. We sent a healthy person to this doctor as a test - they came out with a little bottle of glycerin.
That's not to say it's not merely ADD, but it does sound worrying.
Some things to consider - check out the Pervasive Developmental Disorder (PDD) questionnaire on www.childbrain.com. Browse around the whole site and see if you find anything familiar. But do the Pervasive Developmental Disorder (PDD) questionnaire (it's not officially diagnostic) and whatever the result, print it out and take it to the appointment. because what it DOES do is help list the things that concern you, and to what extent they seem to be a problem. Add any notes and concerns of your own not addressed by the questionnaire. What the thing does is trigger ideas and thoughts about your child that you otherwise might not think to mention.
Also, read "The Explosive Child" by Ross Greene. One thing that has changed our lives - we no longer punish difficult child 3 for things he says out of impulse. We still correct him, but when he's in a position to listen, not when he's raging. There is a lot of useful info in that book. Once you start punishing for angry bad language it begins to look like that scene with the nun in "Blues Brothers" - ludicrously funny, if you're not the parent of the child.
I know people can recommend books a lot, so Google the book first, trawl Early Childhood forum here (or the archives) for some useful inside glimpses of the book; get it from the library before you commit to buying yet another book. But a lot of people on this site recommend it. With good reason.
There are other similar books on bipolar - I'm less familiar with bipolar so I can't give you much advice there, but check out everything that seems familiar, take notes, take him to another independent expert (a REALLY independent one) and see how you go.
Marg