Hi, Jenn! First, I am so sorry your family is going through this. I know how hard it is to be the parent of extremely difficult kids - personally, I often feel that others are judging me as a bad parent b/c my kids are incapable of controlling themselves.
My just-turned-8 year old son was dx'd with ODD when he was 3 years old. I knew from infancy that he was *different* - extremely strong-willed, determined, driven. His behaviors really became much worse between 2 and 3 - far beyond the 'normal' Terrible Two's. I had him evaluated by a neuropsychologist and was shocked with the ODD diagnoses at such a young age. When he was in kindergarten, he was also diagnosed with ADHD. We held off on meds for a bit, but finally gave in to try and get my son some internal peace. That was 2 1/2 years ago. About 6 months ago, he was hospitalized in-patient at a children's psychiatric hospital and received an additional diagnoses of 'mood disorder - NOS' - basically, bi-polar disorder for kids. He's on a couple different meds and it has truly helped in so many way - especially with his anger control. Now, he still gets angry, but he's able to pause and do his breathing techniques or to verbalize what is upsetting him instead of going straight into a rage.
My step-son has also been dx'd with ADHD but I do not believe that is his problem. I firmly believe (and we've got neuropsych testing scheduled at the end of October) that he is on the autism spectrum, most likely with high functioning Asperger's.
Anyway, I agree with suggestions to get testing done. It's very, very easy for people who are NOT living with this kind of child to say, "Oh, gosh, that's perfectly normal. ha ha ha" You are his mom. NOBODY knows him better than you. NOBODY can judge what you have seen or lived through as the parent of your child. Seek additional help and, if you do not like what one psychiatrist says, go to another. Ask around in your area to find out who the REAL pro's are in that field.
I don't know where you live, but my SS is going to Dr. Richard Dowell in Lewisburg, PA for his neuropsych testing. We keep hearing his name from other parents every time we discuss my SS's behavior issues. He's testified before the PA Supreme Court as an expert and, from what I hear, he is THE MAN when it comes to determining what is *truly* happening with kids when other docs have totally missed it.
Trust your instincts and read as much as you can. Two of my favorite books to help me with my son are: "Easy to love, Difficult to Discipline" by Becky Bailey and "Try and Make Me!" by Ray Levy and Bill O'Hanlon (both available on Amazon). Good luck and keep reaching out for others who really understand what you're going through!!