M.,
Well, first, let me assure you... I have BEEN THERE, DONE THAT, bought the t-shirt, and went back for seconds!
I have 2 little boys, ages 6 and 3 1/2 (he'll be 4 next month). My 6-year old went through a similar situation around age 2 when little brother was born. It was easy to chalk it up to 'typical' 2-year old behavior and some jealousy over not being #1 and only anymore. It continued to age 3 and into age 4, but we were lucky enough to find some caring and support through the Florida Center here in Sarasota. He still has plenty of 'issues' at 6 and in Kindergarten, but we survived and learned how to deal.
WELL. Child #2. He started with the similar behavior just last March after being 'ideal' as a toddler. Other than a biting phase that got him politely asked to leave a daycare center at age 16 months, he had always been fairly laid back and happy. The behaviors started with some defiance of adults and some problems interacting with kids... including the friends he'd been just fine with for months. It progressed to outright defiance of authority and a need to just BEG for negative attention. He left that preschool before he was kicked out (we saw the writing on the wall!). We are also going through Florida Center with him, where he sees a therapist and has already given us some remarkable insight into what may be going on in his little head and heart. There is a lot of anxiety and fear that we just didn't see. Even after 1 child who had similar problems and my experience in child development and behavior (I have a master's degree in social work), we still felt helpless to get to the core of the problems. We are seeing a lot of sensory issues (also the case with my oldest) and a lot of anxiety. He is in a very small preschool now with a different kind of setup and with a different kind of structure. He seems to be thriving. They 'enjoy' his quirks and seem to handle his need to be the know-it-all, smarter-than-thou teenage attitude. (By the way, he has FOR MONTHS told EVERYONE that he is 16. REFUSES to say he's 3... and he DOES act like a defiant teenager which is often VERY amusing). The school has a great attitude about kids and that they each have different needs. He does not even have to take a nap if he doesn't want to (he's allowed to sit in the office or library and read or draw), which has eliminated a TON of the anxiety that perpetuates a lot of his negative behavior. The director is AMAZING. She just 'sees' that kids have different needs and, since the program is so small, is able to accomodate.
My point is this... I have BEEN THERE! I would be happy to chat with you about Florida Center, the preschool my son attends (there are actually openings in his class... the same class your son would be in if he were to attend... so if you're interested I can give you more information), or just a "been there" pat on the back. Hang in there. You obviously care very deeply about your children, and your son knows that. Things have a way of working themselves out.
Good luck!
T.